As Sean Donlon's star-turn as Ambassador of Ireland was not like anyone else's, his leave-taking has been out of the ordinary.

The other night, at one of many "wakes," this one at the home of Andrew Mulligan, a former captain of the Irish Rugby Team and press officer of the Common Market, guests were piped in by torchlight to the strains of "The Minstrel Boy." Inside, a harpist plucked an ancient lament, duets were sung in Welsh and Irish. There were recitations, imitations and choral singing directed by the ambassador, who was toasted as "a peacemaker." It was not your typical Embassy Row farewell.

Donlon, who is a mirthful 41, was involved in American politics up to his bushy black eyebrows. He had no example to follow. His predecessors moved about building good will, talking up trade and tourism. Donlon led a crusade against terrorism in Ireland.

He went to Hibernian halls and neighborhood bars, where Irish-Americans, burning with memories of 1916, yelled "Up the IRA" and "Brits Out" and he firmly told them they were wrong.

He went head-on against Noraid and the Irish National Caucus, providers of funds for the fray. He took on Rep. Mario Biaggi, the Democratic congressman from the Bronx who chairs the House Ad Hoc Committee on Ireland. At one point, Biaggi proposed a "Peace Forum" at which IRA gunmen would be invited to testify.

"These people," said Donlon, "are helping those who are murdering Irishmen and Irishwomen." His activities naturally incurred the enmity of Biaggi, who lodged complaints with the former prime minister, Charles Haughey. In the summer of 1980, Donlon was summoned home and told he must make his peace with Biaggi or lose his job. Donlon declined.

In an unprecedented intervention, House Speaker Thomas P. (Tip) O'Neill Jr. and Sen. Edward M. Kennedy, sprang to his aid. They called Haughey in Dublin in the midde of the night and warned him that the dishing of Donlon would bespeak an abandonment of the moderate course. The speaker reproachfully told the sleepy prime minister that "Sean is a beautiful fellow." Haughey backed down.

Last June, Donlon's good friend, Garrett Fitzgerald, was narrowly elected prime minister, and soon after, Donlon was called home and offered the highest post in the foreign ministry, that of secretary of the department.

He goes back at a moment that is as promising as any in the last 10 years of bitterness and mayhem. It is true that the hunger strike, which unleashed a flood of worldwide sympathy--and funds--for "the boys" is over. The IRA has returned to its loathsome violence. The most recent example: an Ulster policeman was shot dead outside a Belfast maternity home as he was waving goodbye to his wife, who had just given birth.

What is hopeful is that Fitzgerald has taken a historic first step toward the possible reconciliation of North and South. Speaking on Irish radio, he addressed the deep fears with which Ulster Protestants regard unification.

"If I were a Northern Protestant today, I cannot see how I could be attracted to getting involved with a state which is itself sectarian. Our laws and our constitution, our practices and our attitudes reflect those of a majority ethos which are not acceptable to Protestants in Northern Ireland."

He was conceding that the republic is a theocracy--birth control and divorce are forbidden--and admitting that it must change if peaceful progress toward unification is to be made.

He is the first Irish prime minister to give such assurances. The Economist of London called his move "wildly daring"--he risks the wrath of Ireland's powerful hierarchy--and the initial response from Northern Ireland has been positive. The head of the Orange Order, the leading Protestant organization, the Rev. Martin Smyth, said, "Fitzgerald is at least opening up the debate"--which for him was a very great deal to say.

In his new post, Donlon will have principal responsibility for the North.

The Anglo-Irish talks begun between Haughey and British Prime Minister Margaret Thatcher will be resumed before the end of the year.

The American connection could be vital.

The hope is that Ronald Reagan, who boasts of being Irish and twice visited the embassy, will use his influence with Mrs. Thatcher, his fiscal soul mate. He could help persuade her to find a political solution to the 700-year-old Irish problem.

The president has put his friend, William Clark, the deputy secretary of state, in direct charge of the Irish question--we still, after several embarrassing starts, have no ambassador in Dublin.

Clark will visit Ireland in December.

Donlon leaves behind a political network he helped to set up among pals on the Hill, the Friends of Ireland, with 100 members from both parties and House Whip Thomas S. Foley in charge.

They hope he'll be able to repeat at home the brilliant success he had here in changing people's minds. They know that in Ireland, it's much harder.