Another convicted Irish Republican Army guerrilla joined the hunger strike at Northern Ireland's Maze Prison today amid indications that the IRA pressure tactic, having already claimed two hunger strikers' lives, may be eroding moderate support for British Prime Minister Margaret Thatcher's unyielding policy.

Thatcher reiterated her refusal to give political prisoner status to IRA inmates in the British-ruled province in replying to a telegram from prominent U.S. Irish Catholic politicians urging her to be more flexible.

"You question a 'posture of inflexibility' that must lead inevitably to more violence and death," she wrote to Sens. Edward Kennedy (D-Mass.) and Daniel Moynihan (D-N.Y.), House Speaker Thomas P. (Tip) O'Neill (D-Mass.) and Gov. Hugh Carey of New York. "But that is not the government's posture." She then pointed to her government's decision to allow visits to the hunger strikers by the European Commission of Human Rights, a papal envoy and politicians from the Irish Republic.

She also said offers to the IRA prisoners of improvements regarding clothing and work had no effect on the prisoners, "whose sole purpose is to establish a poltical justification for their appaling record of murder and violence -- murder and violence which deserve the same total condemnation in Northern Ireland as they should get in the United State."

Other religious and political leaders in Northern Ireland and the Irish Republic have made similar appeals for greater flexibility and now express fears that the apperance of British intransigence in the face of further deaths of hunger strikers could alienate moderate political opinion on both sides of the border.

"British policy is allowing the IRA to show the Thatcher government as simple-mindedly stubborn rather than principled," an Irish diplomat said here today. "The British have failed to forsee the political effect of the hunger strike -- it is boosting the IRA while cutting the ground away from the moderates in Ulster."

Acting on the IRA pledge to replace each hunger striker who dies with a new protestor, IRA prisoner Brendan McLaughlin, 29, stopped eating today, following the death Tuesday of IRA prisoner Francis Hughes after 59 days without food. McLaughlin, serving 12 years for firearms possession, joined three other IRA prisoners already on strike, two of whom have gone without food for 54 days and whose condition is described by British officials as "deteriorating." The third prisoner already on strike replaced hunger striker Bobby Sands, who died last week.

IRA sources claim that between 70 and 100 prisoners have volunteered to go on hunger strike if necessary in the Maze Prison.

Tensions surrounding the IRA tactic to force the British to treat them as political prisoners rather than common criminals have led Prime Minister Charles Haughey of the Republic of Ireland to postpone twice an impending general action in which the close relations with Thatcher will almost certainly become an issue. The IRA is outlawed in the Irish Republic as well as in Northern Ireland.

Two days of street demonstratoins in Dublin in support of the hunger strikers led last night to the most violent clashes between Irish police and demonstrators seen in the last seven years.

Irish officials said today that the IRA appeared to be seeking with some success to isolate Britian politically, forcing the British government to take a hard stand on the hunger strike that could appear to Ulster's Catholic minority as a tilt toward hard-line Protestant opinion.

The officials' concern with the possible erosion of moderate political opinion and the coming Irish election appeared to underlie a statement two days ago by Haughey that "no Irish government can be indifferent to the prospect of these deaths continuing," and that Northern Ireland as presently constituted "is no longer a viable political entity."

Irish Foreign Affairs Minister Brian Lenihan met today in Strasburg with representatives of the European Commission on Human Rights in an effort to find a new way to bring them into a possible settlement of the hunger strike. Commission representatives were unable to act in Sands' hunger strike after he refused to sanction their intervention.

Haughey has met with relatives of two of the hunger strikers, and Irish sources said today the commission might suggest small changes in prison procedures that the British government might be able to accept without appearing to give in directly to the IRA demands. But so far the IRA has rejected minor alternations short of granting political status for its imprisoned members.