The 15-year war between British, Irish and Ulster security forces and the outlawed Provisional Irish Republican Army (IRA) has drifted into a deadly new phase in which the casualties are going down while the stakes are going up.

The statistics on the wall of the Royal Ulster Constabulary command post here tell one part of the story.

They show a steady downward trend in recent years in the toll from an IRA terrorist campaign -- and from smaller, Protestant terror groups -- that have taken 2,400 lives and wounded 25,000 others since the current chapter of "the Troubles" came to British-ruled Northern Ireland. It began in 1969, when violent Catholic-Protestant clashes brought British troops to keep order.

The new higher stakes phase of the battle, however, is dramatized by the demolished hotel in Brighton, 300 miles from here on Britain's south coast, where an IRA bomb narrowly missed killing British Prime Minister Margaret Thatcher and most of her Cabinet three weeks ago.

The attack was unprecedented in its scope. Although Britain says it is tightening its security, it is confronting an enemy that gradually has changed its strategy and organization and now may be even more elusive.

What emerges from interviews with police and military authorities in Britain, Ireland and here in the six counties of Northern Ireland, and with sources close to the IRA, is a picture of a terrorist organization that looks as though it is being worn down but that actually may now be better structured to carry out the gravest political violence.

While the IRA has been a dull pain for the British for 15 years, suddenly there is an acute need for Britain to contemplate its next moves.

The IRA has vowed that it will try again and will not quit until Britain is driven out of Northern Ireland.

The British government for its part has no intention of leaving. It will not abandon the 1 million Protestants who form the majority in the province and who remain either loyal to Britain or do not want to live under a Catholic-dominated united Ireland.

But both sides say they have no doubt that the IRA will fight for another generation.

"They absolutely believe they can drive the British out and that the stiff upper lip of British resolve is a superficial talent," said one experienced Ulster police official.

"It's like Palestine," a source close to the IRA says of the British. "They never withdraw until they are forced to."

Perhaps the key factor now complicating efforts to track down the IRA is that in recent years it has reorganized into small, relatively self-sufficient "active service units" or cells that may contain as few as six or seven members, who may or may not know each other before they are ordered into an operation.

The idea, according to both police and IRA sources, is to create separation between a cell and other IRA elements so that if members of one cell are caught, their capture cannot lead to more arrests. If the cells are in Britain, the idea would be to live without support of the Irish community there to reduce the risk from informers.

Only one or two members of the ruling IRA council that issues orders may know precise details of a mission, and there are said to be further "disconnects" along the chain of command to protect higher-ups from being implicated. It was the threat to the IRA in recent years from terrorists who turned informer that prompted the reorganization.

British officials have speculated that just such an IRA cell, which had been lying low for years in Britain, planted the Brighton bomb, possibly weeks before it was scheduled to explode, using a sophisticated timing device.

But other police sources here with long experience believe the bombers may well have come from here shortly before the attack, entering through another apparent hole in the defenses against terrorists. That is the ability to send terrorists rather easily across the border from Northern Ireland into the Irish Republic. From there, they can fly to and from Britain with no passport checks instead of risking the tight security maintained on the Belfast-London route.

Police estimate that there are probably 300 to 400 "hard-core" IRA guerrillas, the ones carrying out the terrorist operations and in the command structure. The vast majority are here in the north, with others in Ireland and Britain.

The typical guerrilla is unemployed, comes from the grim Catholic ghettos around here or Londonderry or Armagh, has grown up in the climate of violence since 1969, and undoubtedly has been "messed about" by British troops, the overwhelmingly Protestant Royal Ulster Constabulary or the paramilitary Ulster Defense Regiment.

A few are described as intellectuals or ideological zealots. But most are said to be driven either by a hatred of the British or of loyalist Protestants, or by a desire for adventure or a taste for violence.

Although more than 2,000 are in jail here charged with IRA-linked offenses, and hundreds more are in Irish and British jails, military officers say the ones still at large are "the smartest ones."

The IRA has proved to be hard to infiltrate. Police say the organization is now obsessed by security, a factor in reducing the number of attacks it can carry out.

Beyond the hard core there is what intelligence officials call "a friendly sea to swim in." This includes circles of sympathizers, couriers, people who provide safe houses, benevolent societies and other organizations without which police believe the IRA would be much more vulnerable.

Those who help may number between 1,000 and 2,000. But officials say the degree of potential support is reflected in the 93,000 people in Northern Ireland -- 13 percent of all voters -- and the 55,000 people in the Irish Republic -- about 5 percent of voters -- who cast ballots for Sinn Fein, the legal political wing of the IRA, in the European Parliament elections in June.

Another key and recent strategy shift is that the IRA is now said to be using a more selective targeting system. They are striking primarily at what they consider to be "legitimate" military, political and economic targets in Britain and the North, as a Sinn Fein official put it, rather than the kind of all-out terror campaigns that blew up stores and restaurants in the mid- and late-1970s.

A turning point was the bombing of Harrods department store in London on Dec. 17, 1983, which killed six persons and injured 91, including some Americans.

Gerry Adams, the Sinn Fein leader, said afterward that the attack had not been authorized, and it was viewed as a public-relations mistake by some of Sinn Fein's leadership.

That is why there now is the prospect that the remaining IRA, organized into harder-to-crack cells, may be turning most of its attention to the high-visibility political targets. Police are especially worried about Britain's royal family.

In the North, the targets now tend to be British soldiers and off-duty Protestant police rather than downtown shops and pubs.

Furthermore, IRA sources say that as experience with the cell structure has grown, authorities in Britain have had little success in catching them.

Scotland Yard reports that it has solved about 75 percent of the IRA-related crimes committed on the British mainland since 1972. These have resulted in the deaths of 85 persons and injured 1,400.

But no one has been charged yet in the Harrods bombing, although three people who may have had something to do with it are in custody. Nor have the attackers been caught for the 1982 bombings in London's Hyde and Regent's parks that killed 11 soldiers and wounded 59 others. Scotland Yard now also has the Brighton case on its hands.

The assassination of British member of Parliament and close Thatcher political associate Airey Neave by a car bomb in 1979 -- carried out by a much smaller and more extreme splinter of the IRA, the Irish National Liberation Army -- also remains unsolved.

Police believe the INLA, openly Marxist and more violent than the IRA, has only a few dozen hard-core members and that the IRA may find it convenient to have a group even more extreme than itself but known by a different name.

Britain is now tightening its security and has set up a new intelligence committee. But there had been a general alert against an IRA attack somewhere in Britain just before the Brighton bombing, and numerous intelligence experts say British security clearly did not measure up. Furthermore, there have been press reports of bickering between security agencies that lead to bungling of arrests, and doubts expressed about the value of the new committee.

Ulster police, who are closest to the situation, believe, however, that the new IRA strategy of more selective attacks may be causing dissension within the organization between the political and the more militant leaderships.

Under Adams, Sinn Fein has made important political inroads in the 500,000-strong Catholic minority in the north. This has been due in part to its effective community work, especially with youth and antidrug programs.

If that party is to become the predominant political voice against the more moderate parties, such as the Social Democratic and Labor Party, it is reasoned by some Sinn Fein leaders that indiscriminate violence should be reduced. But extremists within the party may not be content with that, and police say it is not clear yet if the new strategy will hold.

Adams, 35, is a symbol of how sophisticated the Sinn Fein-IRA challenge has become. The bearded, tweedy, socialist party leader provides a less menacing face for the link between the two organizations.

While he denies belonging to the IRA, British, Irish and American officials say he is a former top official of the IRA council and that nothing of significance militarily happens without his knowledge.

"So why don't we arrest him?" an Ulster police officer asked rhetorically. The dilemma, he said, is the same as that confronting any police chief in a big American city who knows who the real crime czars are but can't jail them. "We can't prove it. It's the difference between knowing and proving," the officer said.

Police and intelligence sources say there is no doubt that the main source of weapons for the IRA is the United States and that money from the New York-based Irish Northern Aid Committee (Noraid) is a major factor in buying arms.

Even more money, they say, comes from bank robberies in the North and in Ireland, and from extortion, rackets and tax evasion.

All officials believe that the new IRA strategy is linked closely to Noraid -- and to U.S. gun dealers who take a big chance in arranging shipments -- in terms of continually proving to its supporters that it can attack the British while toning down the more indiscriminate bombing.

Noraid, which is openly sympathetic to the IRA, denies U.S. and Irish government charges of gunrunning and insists that the funds it raises go solely to help the families of IRA prisoners.

Police officials also say that while the IRA has fraternal contacts abroad, and gets some arms in Europe and the Middle East, these links tend to be exaggerated. The IRA is "essentially an indigenous terror group," Ulster police say.

The recent seizure of seven tons of IRA arms en route by sea from the United States, and the less publicized but significant discovery of large quantities of sophisticated bomb timers outside Dublin two weeks ago, have dealt the IRA serious setbacks. But the quantity of these finds also discloses the scope of the ambition of the organization.

Cooperation between the FBI, the Irish police, Scotland Yard and the Royal Ulster Constabulary is said to be excellent by all services.

Those are the forces, including 9,500 British troops here (down from a peak in 1972 of 22,000), arrayed against the IRA. They are formidable, especially the 8,000 members of the Royal Ulster Constabulary who are the most experienced and the ones most on the front line.

But politicians and police say there are two important obstacles to greater success.

One is simply the openness of British democracy. This will always give the terrorists a powerful advantage on the mainland unless Britain decides to begin sacrificing much of the openness it treasures.

The second factor, however, is much more important and is part of the seemingly intractable problem of Northern Ireland and the intense religious animosities that fuel the divisions here.

It is what government leaders in Ireland, moderate politicians here and elsewhere call the growing alienation of the Catholic minority.

Overall unemployment in Northern Ireland is about 22 percent, the highest in the United Kingdom. But as a group of U.S. Catholic bishops reported last week, unemployment among Catholics runs to 40 percent, 50 percent and even 60 percent in some areas. There is evidence of job discrimination as well as a sense in the Catholic community of being battered by the police and justice system. Sentiment is widespread that Britain does not pay enough attention to them.

The difference between Catholic and Protestant neighborhoods is visible to anyone touring Belfast. Despite efforts at building new housing, many Catholic areas are sealed-in slums, grim breeding grounds for new IRA recruits and growing Sinn Fein support. Even the new housing complexes are designed in a curious way, with only one road in and out, leading many Catholics to believe they are designed so they can be quickly sealed off by security forces in the event of trouble.

Reversing this alienation, as the U.S. bishops and many politicians have said, is going to take a political solution involving Britain, Ireland and Northern Ireland.

Despite proposals for reconciliation schemes published in May by the New Ireland Forum of moderate Catholic parties from Ireland and the North, there is no solution in sight that would calm the fears of the Protestant majority, or stem the determination of the IRA