Last month's IRA assassination of British Lord Mountbatten has bruised Ireland's national psyche, shaken its government, rekindled controversy about Irish unity, strained relations with Britain and cast a shadow over the coming visit of Pope John Paul II.

Two days of bomb scares this week have reinforced widespread fears of violence erupting during the papal visit, perhaps the most important event in Irish religious history since the missionary work of St. Patrick 15 centuries ago.

The murder of Mountbatten, a British war hero and royal elder statesman, by Provisional Irish Republican Army terrorists forced the Vatican to decide against allowing the pope to cross the border into British-ruled Northern Ireland. It is there that the archbishop of Armagh serves as the primate of all of Ireland.

Officials here still worry that Ulster Protestant parlimentary groups may try to retaliate for Mountbatten's murder through some sort of violence when Ulster Catholics stream across the border to see the pope on his visit to Drogheda, just 30 miles from Ulster.

"This is tribal warfare," said Irish historian Ronan Fanning, a University College of Dublin professor. "Mountbatten was one of their revered tribal chiefs. That's what made it so different from the killings of soldiers or even the British ambassador here (in 1973). Now, the highest chief of the other tribe is coming."

Mountbatten's murder has been blamed for the disappointingly small number of British Catholics who thus far have made arrangements to travel here for the Pope's visit. There also has been less interest than expected from elsewhere in Europe, although thousands of Poles still are expected to make the pilgrimage.

In all, Catholic Church officials here expect as many a 2 million people -- equivalent to nearly two-thirds the population of Ireland -- to turn out to see the pontiff. The pope will make a stop here and in five other locations in Ireland during his tightly scheduled three-day weekend visit, Sept. 29 through Oct. 1. More than 2,000 journalists and technicians are being accredited to cover the events here, which will be broadcast to the world via Irish television.

Ireland will be in the global spotlight, and the pope's visit is widely expected here to take on more than the pastoral significance sought by the Catholic Church.

"After Mountbatten," Fanning said, "everyone will be watching for a strong denunciation of the violence. This pope, more than any other recently, has shown himself to be a leader in world affairs."

Until the Mountbatten assassination, Ireland for several years had been less preoccupied with the fate of the six predominantly Protestant northern counties ruled by Britain. Sectarian violence there had subsided, and the British and Irish security forces and courts had succeeded in putting large numbers of IRA terrorists behind bars.

Ireland was much more involved in its own economic and social evolution. It was enjoying an enormous increase in prosperity since joining the Common Market in 1973, modernizing its farming and beginning its own industrial revolution, primarily with new plants making Common Market products for American and European multinational corporations.

The quest for the ultimate unification of Ireland seemed to have been put aside. Instead, political debates centered on how to manage the new economic growth, protect the environment, reduce the high taxes paid by industrial workers, balance growing management and union power, and cope with the sweeping social changes -- such as equality for women -- spurred by the new prosperity and increased contact with the outside world.

The Fianna Fail Party, which had long been dominant in Ireland because of its strong and successful nationalism, returned to power in 1977 with its largest parliamentary majority ever. That success was attributed primarily to Prime Minister Jack Lynch's bold promise to expand the economy and to reward increasingly productive workers with tax cuts.

But after a singularly successful year in 1978, the Irish economy has slipped this year. It has been hurt particularly by rising oil prices and long strikes by postal, telephone and other government workers. The economic growth rate has fallen, and inflation and unemployment have risen.In the most dramatic demonstration here since Ireland won full independence from Britain, an estimated 150,000 workers marched through Dublin last spring to protest Lynch's failure to cut their taxes while continuing to allow farmers special tax breaks.

Lynch, a cautious quiet-spoken balding man with soft, blue eyes who was a compromise choice to lead Fianna Fail before becoming prime minister in 1966, was seen by his critics as losing control of events. Now 62 and still suffering from a leg injury from a boating accident a few years ago, Lynch was expected to retire long before his government would have to face elections in 1981 or 1982.

Lynch was thought to be waiting only until after the November Common Market summit here, which climaxes Ireland's turn in the presidency of the European Economic Community, a visit with President Carter in Washington later this year and discussions with British Prime Minister Margaret Thatcher about a new British political initiative in Northern Ireland.

Lynch also is known not to be eager to be succeeded by the leading contender in Fianna Fail, the charismatic health and socil services minister, Charles Haughey. Haughey was tried and acquitted in the early 1970s, while Lynch was prime minister on charges of conspiring to import arms into Ireland illegally, allegedly for the IRA.

Then came the Mountbatten assassination last month. Lynch was criticized by the press and politicians because he did not return from his vacation in Portugal until three days after the bombing, touching off a Cabinet crisis here. Then the British press and government appeared to blame Ireland and called Lynch on the carpet for not cooperating sufficiently in anti-IRA security measures.

Irish government officials were alarmed by suggestions that British soldiers be allowed to hunt terrorists inside Ireland and that Ulster police be allowed to question suspects there. But Lynch agreed, at a meeting with Thatcher after Mountbatten's funeral, to tighten security on Ireland's side of the border with Ulster.

He also took the surprising step, praised as "courageous" by many government officials here, of telling Thatcher he would not insist on immediate, formal assurances of the eventual unification of Ireland if the British could create a home-rule administration in Ulster in which the Catholic minority and Protestant majority would share power.

This drew fire from Sile de Valera. De Valera, 24, is the granddaughter of Fianna Fail founder Eamon de Valera, who steered Ireland to full independence from Britain while heading Ireland's government on and off, from 1932 until 1959. She has capitalized on her grandfather's name and the single issue of Irish nationalism and unity to become the youngest member of the Irish Parliament and Fianna Fail's only elected representative in the European Parliament.

In a well-publicized speech last weekend at a memorial service for an Irish independence hero, she castigated Lynch's power-sharing proposal for Northern Ireland as a "half measure" that "can only serve to exacerbate and fester the problem."

Any "lasting solution," she declared, requires a single Irish government "to decide the affairs of the whole nation. Any transitionary steps that do not include this principle betray the beliefs and hopes of all those patriots who have died for that cause." She sought, she said, "not the freedom of a geographical fragment of Ireland, but the freedom of all Ireland, every sod of Ireland."

Taking De Valera's challenge quite seriously, and believing, according to knowledgeable sources, that she was acting on behalf of a sizable minority of hard-line opponents inside Fianna Fail, Lynch issued an immediate rebuttal. He also called an emergency party meeting shortly before the papal visit at the end of this month. Irish officials have expressed concern that this rift could weaken Lynch's hand considerably in "delicate negotiations" still to come with the British on border security measures and an eventual political initiative in Ulster.

As officials here point out repeatedly, the Provisional IRA is seen to be as much of a threat to Ireland's government, which it regards as a kind of "Vichy" rule, as it does to the British and the Protestants in Ulster.

The Mountbatten killing was an alarming reminder that the IRA can and will strike inside Ireland, no matter what embarrassment and difficulty it causes here, according to government officials. This increases their concern about the pope's visit.