A police car of the Irish Garda followed by an armored personnel carrier with Irish Army troops cruises slowly along a narrow country road here just inside the Republic of Ireland's border with British-ruled Northern Ireland.

Police officers scan the rolling green fields and hedgerows for unusual activity. Random roadblocks are set up for an hour or two, checking cars crossing the otherwise completely open border between Ireland's 26 counties and the six counties of Northern Ireland.

The border frequently has been a beckoning escape hatch for Catholics of the outlawed Provisional Irish Republican Army fleeing south from Ulster's Protestant-dominated police or British Army troops.

If they make it across, which is not hard, they probably can fade into the population for a long time, police say.

The Republic of Ireland and the border region have become crucial factors over the years in the long battle with the IRA.

Ireland's prime minister, Garret FitzGerald, is widely viewed as the Irish leader most committed to ending IRA violence and seeking to reconcile north and south.

At considerable political risk, he has vowed cooperation with the British in security matters, which is not easy for Irish politicians, and he has spent heavily on security forces.

He has chastised Ulster's police for losing the confidence of Ulster's minority Catholic population.

Earlier this year, Irish courts for the first time allowed two alleged terrorists, Dominic McGlinchey and Seamus Shannon, to be extradited to the north rather than accept their claims of involvement only in political offenses.

And it was Irish police who caught the IRA terrorists who, in 1979, planted a bomb on a boat and killed Lord Mountbatten, uncle of Queen Elizabeth II.

Fifteen years ago, there were no Irish troops or posts along the border. Today there are 2,000 troops and 11 permanent outposts. But this border is 280 miles long, has 251 road crossings and remains relatively easy to cross. If a terrorist does not want to drive, he can take the train and not have to show a passport.

Irish authorities believe there has been a reduction in border activity, although it is hard to prove. Incidents of IRA supporters coming up to the southern border and firing across at British Army observation and infantry posts just across the road have been reduced. But antiterrorist specialists believe much of the decrease in activity along the border is a reflection of the general decline in the number of IRA incidents in recent years.

The ones that do take place are increasingly tricky. Last month, just a few miles from here in South Armagh in Ulster, the IRA hijacked an oil truck, abandoned it in a cul de sac near the border and planted land mines around it in the hope of blowing up approaching British troops. While the Irish sealed off roads from the south, the British photographed the truck from a helicopter and studied the situation for a week before sending in demolition teams to dig out the trap set for them.

Despite the Irish Republic's efforts at cooperation with Northern Ireland and Britain, most security officers say Ireland still presents a big gap in the war on the IRA.

Because the province has been on the front line, British Army and Northern Irish police forces are heavily armed, have the latest equipment and broad powers of arrest. In Ireland, the police are lightly armed, if at all, and the Irish Army is totally under police control. This means that any call for help in pursuing a fleeing terrorist from the North must first go to the police who must then ask for Army help. There is no Army-to-Army communication.

Second, the specialists say, Ireland has a growing crime problem in the cities because of burglaries, bank robberies and drug traffic, and that siphons off much of the police effort.

Finally, despite the recent court decisions, Northern Ireland authorities claim it is extremely difficult to get an escaped terrorist back from Ireland through extradition, and the IRA knows this.

"Psychologically, they still regard the republic as an escape hatch," said one experienced officer in the North. "Even if they don't use it, just the fact that it is there is important. They know if they can get down there and keep their hands clean, it will be a difficult and tortuous procedure to get them back."