MARGARET THATCHER, the British prime minister, meets today with Jack Lynch, her Irish counterpart, to urge him to take certain steps that will make is easier for the British and Northern Irish authorities to counter terrorism: hot pursuit across the 300-mile unmarked border, extradition of suspects sought for crimes in Britian or Ulster, and so forth. It sounds reasonable-- an extension of the example Mrs. Thatcher set by visiting Northern Ireland after Lord Mountbatten was assassinated and 18 British soldiers killed last week. That tour answered a clear public need to show that the political leadership is not intimidated by terror.

But the new steps sought by Mrs. Thatcher may not be so reasonable. Ireland's anti-terrorist performance, both in terms of laws on the books and the energy of its officials, does not support Mrs. Thatcher's implicit premise that Ireland has lagged in this regard. Nor does Ireland's record support the offensive suggestion sometimes made by others that considerations of Catholic solidarity are to blame.

Perhaps there are further steps that the Ulster constabulary and the British army could take in Northern Ireland to make their battle against terriorism (Catholic and Protestant) more effective. Mrs. Thatcher's government, which in the absence of Ulster self-rule controls both of those bodies, is reviewing the security needs, and Mr. Lynch should be doing the same. But notwithstanding the emotion of the moment, it would be an error for yet another British government to treat the Irish question as though it were chiefly a matter for the police. This is the gist of the counsel Mr. Lynch plans to offer Mrs. Thatcher today. "Unless we can produce a situation in which the community in the North can give allegiance to a formal administration, then this kind of violence will continue," he observed the other day. "It is a question of getting at the cause of the matter, not the effect."

The cause of the matter lies in the Protestant majority's discrimination against and control over the Catholic minority. The solution offered by Irish Republican Army terrorists, to push the British out and precipitate the forcible unification of Ireland and Ulster, is a formula for bloodshed and civil war. The alternative, painful to seek, difficult to find, is a new political deal ensuring majority rights and minority guarantees in a democratic context. This is the direction in which Britian's friends, in Ireland and in the United States, earnestly hope to see Mrs. Thatcher go. If terrorism is the Irish disease, political compromise-- or rather, political courage-- is the cure.