Irish Prime Minister Jack Lynch has stirred up tempest in a teapot in Britain by speaking out on the supersensitive issue of Ulster's future.

In a rambling radio interview on Sunday, Lynch managed to offend the most cherished beliefs of Northern Ireland's Protestant majority.

He talked of eventual unity between Ulster and the Irish Republic, suggesting that London should "indicate" its "interest" in such a goal.

Lynch, 60, said that Prime Minister James Callaghan had privately assured him that any new Ulster governor would give minority Catholics a share in power. He held out the prospect of "some form of amnesty" for Irish Republican Army convicts if a real peace ever comes to the war-torn province.

These seemingly innocuous and familiar remarks brought angry protests in London and Belfast. Ulster's Protestant politicians broke off the desultory talks they were having with London's representatives over a limited form of local rule.They insisted that Callaghan must repudiate what Lynch said was an assurance over power-sharing.

Roy Mason, Callaghan's minister for Ulster, declared he was "surprised and disappointed by the unhelpful comments made by Mr. Lynch . . . Talk of an amnesty can do nothing but give succor to lawbreakers."

In Dublin, Garrett Fitzgerald, the leader of the opposition Fine Gael party, said Lynchs observations were "ill-considered" and "dangerously" obscure.

A close reading of Lynch's remarks shows he said nothing he has not said repeatedly in the past. If anything, he softened his party's published demand for a British declaration to get out of Ulster.

Tonight, Lynch attempted to reduce the "unexpected" heat his words had generated, stressing that his prop on amnesty would take effect only when IRA "violence and hostilities ceased and were not likely to break out afterwards."

Moreover, he said, he would not pardon those "convicted of crimes of violence . . . for offenses against the state." Lynch said he was continuing Ireland's cooperation with British police and soldiers in their joint war against the IRA.

Political observers are convinced that the shouting and alarums in London have more to do with Callaghan's domestic political considerations than with Ulster itself.

Callaghan is running a minority government that openly depends on uncertain support from 13 Liberals to stay in power. Behind the Liberals, Callaghan has a second line of defense, 10 Ulster Protestant members of Parliament.

Most of them generally vote with Callaghan's Labor government and he has gone to great lengths to keep them happy. For one thing, he and his minister Mason never publicly talk of power-sharing. Most Protestants hate the idea, largely because it would threaten their near monopoly of jobs in the province.

In 1974, Protestant workers paralyzed Ulster to destroy a government that gave Catholics a share in governing. There has been no serious political initiative in the province since then.

The Protestant leaders' boycott of home-rule talks affects very little. The talks were deadlocked anyway. As a minimum demand, Catholic leaders insist on a share in any rule; Protestants are equally insistent on retaining their unfettered supremacy. The present state of affairs, rule by Britain, freezes the status quo -- which suits the Protestants. So Lynch's statements are seen as a good opportunity to break off the meaningless talks.

Lynchs comments on a possible amnesty when a genuine peace is achieved undoubtedly offended Mason, a tough law-and-order man.

But observers here say it is hard to cite any guerilla war that ended without some form of forgiveness for guerrilla terrorists.

Perhaps the most emotion-ridden issue in Ulster is the border. Northern Ireland's million Protestants are mostly horrified at the thought of a forced union with the predominantly Catholic Irish Republic to the south. All leading Irish politicians, including Lynch, agree that Protestants must not be driven into union, the aim of the IRA.

What Lynch did assert was that Britain's "negative guarantee" of a Protestant veto erected a "steel wall" against which Protestants stand and refuse to consider any accommodation. He urged Britain to declare that union is an eventual solution. London, he said, should give "an indication of their interest in the unification of Ireland as an ultimate remedy without any timetable.

A substantial number of non-Catholics in Britain would agree with Lynch, including some of Callaghan's key advisers, but how to get there baffles everyone.

One unsolved political mystery is why Lynch chose to repeat his views now. Some think he is eager to unblock the stalemated Ulster question and stir things up in what is probably his last term in office.

His aides, however, insist he had no plan in mind but was simply responding to an interviewer's questions and is astonished at the fuss he has caused. Irish politicians, even canny ones like Lynch, often speak off the tops of their heads and this may have been another example of a local habit.