Irish Prime Minister Charles Haughey decided today to keep Sean Donlon as Ireland's ambassador in Washngton after being warned that replacing him would harm relations between Haughey's seven-month-old government and leading Irish-American politicians.

Haughey was told that he would upset Sens. Edward M. Kennedy (D-Mass.) and Patrick Moynihan (D-N.Y.), House Speaker Thomas P. O'Neill (D-Mass.) and New York Gov. Hugh Carey if he removed Donlon in response to complaints about him from the Washington-based militant Irish nationalist lobby, the Irish National Caucus, and its ally in Congress, Rep. Mario Biaggi (D.N.Y.), according to well-informed Irish sources.

Kennedy, Moynihan, O'Neill and Carey, known in Ireland as the "Four Horsemen," have cooperated with previous Irish governments on a policy of discouraging Irish-American sympathy and aid for Irish Republican Army terrorism in the British-ruled province of Northern Ireland while urging the eventual peaceful unification of the province with neighboring Ireland. Donlon, appointed ambassador to the United States in 1978 by Haughey's predecessor as prime minister, Jack Lynch, has been a strong advocate of this policy.

He and Lynch also were outspokenly critical of the Irish National Caucus and Biaggi for what they saw as helping the cause of IRA terrorists, particularly by blaming Britain rather than the IRA for the sectarian violence in Northern Ireland.

They tried to discourage Irish-American leaders and politicians from associating with the caucus or with Biaggi's Ad Hoc Congressional Committee for Irish Affairs.

This angered the Rev. Sean McManus, who runs the caucus, and Neil Blaney a prominent, militantly nationalist Irish politician, who demanded that Haughey replace Donlon and pursue a more aggressive policy on Northern Ireland in Washington. Within the past week, informed Irish sources said Haughey was strongly considering moving Donlon to the vacant post of Irish ambassador to the United Nations.

Today, Irish Foreign Minister Brian Lenihan announced that another Irish diplomat, Noel Dorr, the head of the political section of the Foreign Ministry in Dublin, would be sent to the United Nations and Donlon would stay in Washington.

"Press speculation over recent days that [donlon] was to be moved because of a change in government policy over Northern Ireland is entirely without foundation," Lenihan said in a statement. He added that Haughey's government had "complete confidence in Mr. Donlon as its representative in Washington."

The decision to send Dorr rather than Donlon to the U.N. was made only this morning at Haughey's Cabinet meeting, according to informed sources, after Haughey was informed of the "very, very serious concern" of moderate Irish and Irish-American politicians that moving Donlon would be seen as siding with the Irish and National Caucus and Biaggi and "giving aid and comfort to the Provisional IRA and its sympathizers."

Kennedy said in a statement released by his Washington office today that "I am pleased that at this important time in Irish-American relations [Donlon] will continue in his present position. He has been an effective voice in discouraging American support for violence in Northern Ireland, and an equally effective voice for Irish unity as the only realistic avenue to genuine and lasting peace and reconciliation."

Biaggi reacted mildly to today's decision. "The role of any ambassador is to carry out the policies of the government in power," Biaggi said in Washington. "If Mr. Donlon's role as the spokesman for the policies of Prime Minister Haughey has been clarified, then his retention presents no problem.

"I fully support Prime Minister Haughey's efforts to achieve peace, justice and a united Ireland and expect the work of Ambassador Donlon to be complementary."

A spokesman in Biaggi's Office also took issue with criticism which came today from John Hume, the leading Catholic vote-getter in Northern Ireland. Hume said Biaggi and his organization "enjoy no support whatsoever with any substantial section of Irish opinion."

[Calling the Ad Hoc Committee "a significant voice of opinion in America," the Biaggi spokesman said: "The ability to raise the Irish issue in America is the sole objective of the Ad Hoc Committee, and through that it should have an impact on Irish thinking."]