President Reagan yesterday denounced the "moral bankruptcy" of terrorists in Northern Ireland, and urged Americans to refrain from aiding the provisional wing of the Irish Republican Army with money and weapons.

"Some few but vocal Americans believe that differences between Irishmen can only be solved by violence and intimidation," Reagan said at a festive reception at the Irish Embassy. "They are no friends of Ireland. They disgrace the principles for which both Ireland and America stand. I would urge my fellow Americans not to listen to such people."

Reagan, whose paternal grandparents came from Ireland, used his St. Patrick's Day platform to issue the strongest statement he has ever made deploring violence and urging reconciliation in Northern Ireland. He warned that the U.S. government would continue to prosecute those engaged in illegally exporting arms to Ireland, and said that "those who advocate or engage in violence and terrorism should find no welcome in the United States."

The Irish government has launched a "forum for consultations" aimed ultimately at bringing the British government and Northern Irish politicians together to discuss changes in the status of Northern Ireland. Because of this effort and the controversy surrounding the St. Patrick's Day parade in New York, where IRA supporter Michael Flannery was named grand marshal, the Irish government had made known in advance its desire that the president would speak out against American support for Irish terrorists.

Reagan responded as the Irish hoped he would. While he had deplored Northern Ireland terrorism at a St. Patrick's Day reception two years ago, he made no mention then of any specific group of terrorists.

Yesterday Reagan made pointed reference to the murder of Judge William Doyle by the provisional wing of the IRA, and identified himself with a eulogy at Doyle's funeral that said: "We commit ourselves once more to work for peace and reconciliation. Our belief in peace is unshaken. Our hope for peace is irrepressible."

A formal statement issued by the president on St. Patrick's Day was less specific than his remarks, but it also strongly condemned violence. When White House spokesman Larry Speakes was asked at whom the statement was directed, he replied, "He's clearly talking about the IRA."

Reagan chose not to respond during a photo ceremony with House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) when a television correspondent asked him his opinion of Flannery. The 50 guests assembled in a House dining room for a lunch which O'Neill hosted for the president booed, though it was unclear whether they were reacting to the mention of Flannery's name or to the question that interrupted their celebration.

"We're just not getting into personalities," Speakes said when he was asked why Reagan would not discuss Flannery. "There is no need to deal with personalities."

Reagan's stand yesterday was lauded by Irish Foreign Minister Peter Barry, who presented the president with a shamrock at the embassy ceremony.

"The strong condemnation of violence which you've all made and your consistent commitment to stemming the flow of U.S. dollars to the agents of terror in Northern Ireland we heartily applaud," Barry said. "The Ireland we seek is one which will never be achieved by violence."

Barry spoke in a similar vein at the closed-door lunch held by O'Neill, the only serious remarks at a session dominated by an exchange of Irish jokes between the president and the speaker.

O'Neill is one of 48 House members and 26 senators who, as "The Friends of Ireland," signed a St. Patrick's Day statement which deplored violence and said that any lasting political settlement could be achieved "only by peaceful and constitutional means."

The signers included Sens. Edward M. Kennedy (D-Mass.) and Daniel Patrick Moynihan (D-N.Y.), both of whom applauded the president's remark at the embassy yesterday.

Reagan has always made a point of his Irish heritage, and he did again yesterday, saying he was proud to trace his roots to Ballyporeen in County Tipperary. But the president's attention on this St. Patrick's holiday was focused on the continuing violence in Northern Ireland and the need for reconciliation.