Irish Prime Minister Charles Haughey is considering the replacement of Ireland's ambassador to Washington, Sean Donlon, who is perceived to be out of step with Haughey's campaign to unify Ireland and neighboring, British-ruled Northern Ireland.

Militant Irish nationalists in Ireland and in the Irish-American community in the United States have complained that Donlon has discouraged Irish-Americans from associating with U.S.-based militant Irish groups opposed to the British role in Northern Ireland.

Donlon and former Irish prime minister Jack Lynch, who sent Donlon to Washington as ambassador in 1978, criticized these groups for aiding propagandists for Irish Republican Army terrorists in Northern Ireland. The groups blamed British rule rather than the IRA for the violence there and urged this view on Congress and the Carter administration Donlon and Lynch also strongly urged Irish-Americans not to give sympathy, money or arms to the IRA.

Sources in Dublin said Haughey is not responding directly to the complaints about Donlon and opposes IRA violence just as staunchly as Lynch, whom Haughey replaced as prime minister last December. But they suggested that removing Donlon would be symbolic of Haughey's emphasis on pressing for the support of the United States and other countries to help convince Britain that the problem of Northern Ireland can be solved only by eventual British withdrawal and Ulster's unification with the rest of Ireland.

"While telling Irish-Americans, 'Don't support the IRA,'" said one well-informed source outside the government in Dublin, Haughey "wants the Irish ambassador [in Washington] to say, 'But do support the Irish government in working for the unification of Ireland.'"

Donlon is expected to be appointed ambassador to the United Nations and be replaced in Washington by the present Irish ambassador to Britain, Eamon Kennedy, according to informed sources in Dublin. Irish government officials insisted that final decisions have not yet been made, but an announcement about the U.N. post is expected soon.

Appointment of Donlon as U.N. ambassador to replace Paul Keating, who died in April, would be a promotion, Irish officials stressed, particularly because Ireland hopes to gain a seat on the U.N. Security Council this autumn. But it is considered highly unusual to move Donlon, 39, after less than two years in an important embassy, especially since he is considered one of Ireland's best young diplomats and an expert on Northern Ireland, a key issue in U.S.-Irish relations.

Diplomats in the Irish Embassy in Washington have been as reluctant as officials in Dublin to discuss the impending moves, which were first reported in Dublin by Irish journalists close to the Haughey government.

But Irish diplomats did stress their belief that Donlon had been carrying out Haughey's policy on Irish unification, which Haughey has repeatedly called "the first political priority" of his seven-month-old government.

Dublin sources said, however, that Donlon became too closely identified with Lynch and with his criticism of the Irish National Caucus, a Washington-based Irish nationalist lobby. He also was critical of the Ad Hoc Congressional Committee for Irish Affairs, in particular its chairman, Rep. Marlo Biaggi (D-N.Y.).

This came to a head after a strongly nationalist, independent Irish politician, Neil Blaney, visited Washington at the invitation of the head of the Irish National Caucus, the Rev. Sean McManus, an Irish-born priest, just before Lynch resigned and Haughey became prime minister.

After hearing complaints about Donlon from McManus and congressmen on Biaggi's ad hoc committee, Blaney demanded back in Dublin that Haughey change Ireland's diplomatic policy in Washington and remove Donlon.

"I'm delighted if he's going to be moved," Blaney said today from Strasbourg, where he served an elected Irish member of the European Parliament. "Upstairs, downstairs, sideways, it doesn't matter, so long as he moves."

Haney said Donlon "went out of his way to make clear to Irish-Americans that they shouldn't have anything to do with the Irish National Caucus.

"He also discouraged members of Congress from joining Biaggi's committee, which is the brain-child of the caucus," Blaney added.

The Irish government responded to Blaney's attack on Donlon by saying only that the envoy had been carrying out Lynch's policies, which was seen in Dublin as an indication that Donlon was believed to be out of step with Haughey's policies.

Some observers in Dublin suggested that Haughey may be concerned about being outflanked on the Irish unification issue by Blaney, whose supporters are forming a new political party to challenge Haughey's Fianna Fail Party and the opposition Fine Gael and Labor parties in next year's parliamentary elections.

Haughey, whose popularity has fallen as inflation and recession have sapped Ireland's postwar economic boom, cannot afford to lose any of his party's traditional Irish nationalist support to Blaney, who was the leading vote-getter in western Ireland in last year's elections to the European Parliament.

Before being sent to Washington by Lynch, Donlon was in charge of Anglo-Irish relations in the Irish Foreign Ministry in Dublin, concentrating on Northern Ireland. His work was highly valued inside the ministry, but more militant Irish nationalists in both Dublin and Ulster felt he was too "pro-British" in his approach, according to informed sources.

These sources said that some Irish diplomats are concerned that removing Donlon could send the wrong message to Washington, even though he would be replaced by a respected career diplomat. They fear Donlon's successor will be expected to be more cooperative with Biaggi's ad hoc committee and with the Irish National Caucus.

These Dublin sources also noted that Donlon and Lynch maintained good relations with House Speaker Thomas P. (Tip) O'Neil (D-Mass.), Sens. Edward M. Kennedy (D-Mass.), and Patrick Moynihan (D-N.Y.) and New York Gov. Hugh Carey, who joined them in condemning the IRA and its sympthizers while urging Britain and Ireland to negotiate a peaceful political solution to the Northern Ireland problem. These four influential Irish-American politicians also have kept their distance from Biaggi's ad hoc committee.

The Irish National Caucus publicly disassociates itself from the IRA and says it "speaks for Americans concerned about human rights" in Northern Ireland. It says it does not raise money for the IRA or jailed relatives of its members, as does the New York-based Irish Northern Aid Committee (Noraid), another target of past criticism by Donlon and Lynch.

The caucus does invite to the United States a steady stream of militant Irish nationalists, many of whom Donlon has snubbed.

British officials believe the caucus and Biaggi's ad hoc committee are responsible for influencing the Carter administration to hold up the sale of American-made guns to the police in Northern Ireland, the Royal Ulster Constabulary, which has taken over from the British Army much of the front-line role in combatting IRA terrorism in Ulster.

Dublin sources said that replacing Donlon in Washington would not mean that Haughey was endorsing the views of the Irish National Caucus or Biaggi's ad hoc committee. They pointed out that Haughey's government has cooperated quite closely with the British government in security measures along the Irish-Ulster border. British officials agree with this and have responded by acknowledging in their proposals for home rule in Northern Ireland that Britain, Ulster and Ireland share a special relationship that should be nurtured.

But Irish officials remain skeptical that the British proposals for the Protestant majority and Catholic minority to share power in Ulster under British supervision will win acceptance in the province. Haughey's government believes Britain will eventually have to negotiate a way to unite Ulster with Ireland within a new British-Irish relationship that would allay the fears of Ulster Protestants about becoming the minority in predominantly Catholic Ireland.

This is the vision the Haughey government wants to sell to the United States and European governments so that they will join him in lobbying Britain.