Washington — For years the rumors had abounded that Thomas P. (Tip) O'Neill Jr., chief adversary of the Reagan administration and unabashed liberal Speaker of the House of Representatives, would soon retire. He has finally made it official.
Crooning for reporters the lines ''Sure Ireland must be heaven, my mother came from there,'' the white-haired Massachusetts Democrat confirmed that what he would really want is to be ambassador to Ireland. And even if he doesn't win that post, he will leave the House in 1986.
The announcement ''just came off the top of my hat,'' he said Thursday after the New York Times quoted him on his plans for the future. ''The Irish have a tendency to be loquacious,'' a beaming O'Neill said of himself.
Coming three years before he leaves, the speaker's decision may have set off a long power struggle. However, O'Neill carefully put the mantle on the shoulders of his heir-apparent, majority leader Jim Wright of Texas.
''I love Jim Wright,'' he said. ''He's a beautiful guy. And when I do leave, I'd be very much disappointed if anybody was to oppose him.''
In recent years the Texan has steadily solidified his position to become speaker. His most logical challenger, majority whip Thomas S. Foley (D) of Washington state, made it clear he will not stand in the way.
The selection of the majority leader is ''where I always felt the succession should be,'' Representative Foley said. He also affirmed that ''the speaker's strong statement in support of Jim Wright'' would ensure stability in the Democratic leadership.
With the speaker and majority leader united, he said, ''the leadership should have strong cohesion and direction.''
Representative Wright now appears to be a shoo-in to succeed O'Neill, who has guided the House since 1977. The period has been one of Congress's most contentious in recent memory, including 1981 when Republicans joined with conservative Democrats to hand the speaker a string of defeats.
''We'll be losing a grand champion,'' said Rep. James L. Oberstar (D) of Minnesota, a liberal who credited O'Neill with being ''the Dutch boy at the dike'' in fighting the Reagan administration.
Rep. Buddy Roemer of Louisiana, one of the conservative ''Boll Weevil'' Democrats who have often sided with the GOP, said he was surprised at the announcement, even though he had long expected it. ''He and I disagree a lot, but I really like him,'' he said of O'Neill.
While it is widely assumed that Wright would lead the House Democrats slightly closer to the center of political opinion, Representative Roemer predicted that Wright would not change the course. ''That's in our hands,'' he said of fellow conservatives. ''I don't think who's speaker determines our (conservative) strength or our numbers.''
''Jim Wright has proven that he is a national Democrat,'' said Mr. Oberstar. ''He has taken charge and led fights on the floor many times when it hurt him in Texas.''
''I think he'll be a more effective speaker in rallying the party to the cause (than O'Neill),'' said the Minnesota congressman, adding that Wright appeals to Southern members and has the respect of Northerners.
The majority leader said Thursday of his style of leadership, ''I like to think that I'm in the center of the party. That's where I've always considered myself.''
While Wright has been named as a possible vice-presidential nominee, he said, ''If a genie were to pop out of a bottle'' and offer a choice, ''I think probably I would say 'speaker' because year in and year out . . . the speaker has influenced policy more than has the vice-president.''