BETWEEN THE ST. PATRICK'S Day appeals of two groups of American politicians lies a painful gap -- though perhaps a narrowing one. It represents innocence, on the one side, and responsibility, on the other, in the approach to the dilemma of Northern Ireland.

The first statement is that of Rep. Mario Biaggi (D-N.Y.), a veteran human-rights advocate who founded the "Ad Hoc Congressional Committee for Irish Affairs" two years ago. The Irish American establishment stopped laughing as his committee's membership headed over the 100 mark. Stymied in his attempt to stage congressional hearings, Mr. Biaggi, a self-styled "honest broker," currently hopes to collect all the Irish actors, paramilitary and political, for a "peace forum" in Washington next May.

What is troubling about this is not Mr. Biaggi's ardor but rather his long record of open association with leaders and sympathizers of the Irish Republican Army, a terrorist outfit. There is no reason to suspect that the congressman is acting in anything but innocence and good faith. But though many of his Irish American supporters no doubt think of the IRA in the patriotic terms of 1916, others know exactly how it has changed. Mr. Biaggi's insistence that such charges are propagandistic and unproven has not been enough to clear the air.

His St. Patrick's Day "message of peace," for instance, does address real issues of human rights -- in particular, new allegations that Republican prisoners in Belfast have been victims of official brutality. With the evident intent of fending off charges of patronizing gunrunning, it affirms the 70-odd congressional signers' "total opposition to violence, whether it be institutional or civilian." But the message is essentially a veiled request for British withdrawal from Ulster, and it does serve the IRA strategy of bringing relief to the aggrieved Catholic minority by provoking the unification of Ulster with the Republic of Ireland, which by its strategy could only be violent. This is a recipe for escalating a war that has already produced 2,000 deaths in the past 10 years.

The other St. Patrick's Day message comes from the "Big Four" Irish American politicians -- Hugh Carey, Edward Kennedy, Daniel Moynihan and Thomas P. O'Neill -- and it represents their continuing mainstream effort to bring Irish American and official American influence responsibly to bear. This group has perceptibly bent under the pressure of the Biaggi forces. Gov. Carey, feeling he needed Mr. Biaggi's help in his last reelection campaign, endorsed the "objectives" of the Ad Hoc Committee. Partly in response to the charge of being soft on the British and partly in response to events, Carey and company now lay the new allegations of brutality directly at Britain's door. They also move away from their prior favor for Catholic-Protestant "power sharing" in Northern Ireland, and raise the prospect of British withdrawal and ultimate unification of north and wouth Ireland -- this to be done peacefully, and with Irish consent.

The situation is, then, that the Biaggi group, though disadvantaged by its reputation of providing cover and respectability for terror, is using humanrights issues in a manner suited, if not intended, to advance IRA aims. Carey-Kennedy-Moynihan-O'Neill are regrouping to protect their exposed flank and increasing their emphasis on reaching the Biaggi group's goal (unification) by peaceable means.

It is not, of course, for Mario Biaggi or the Irish American establishment or even for the American government to dictate terms in Northern Ireland. What is important is that all Americans realize that their words and political maneuvers play into the debate and the atmosphere there. For that reason, it is morally and politically vital that the American impact be consistently on the side of conciliation and peace. That is the test the Biaggi group fails.