What Ronald Reagan didn't need, as he tried to dig out of the rubble of his shameful decision in favor of segregated schools, was a delegation from Northern Ireland, where discrimination is the custom of the country, although no longer the law.

The conspicuous beneficiary of the new ruling, is, of course, Bob Jones University of Greenville, S.C., where segregation is a matter of religion. And the most famous honorary doctor of Bob Jones is, of course, the Rev. Ian Paisley, who, if not Ulster's leading bigot, is certainly its ace demagogue.

Paisley never studied at Bob Jones. He didn't need to. Ulster is an open university for intolerance, its Catholic minority having been deprived of opportunity and public office until a civil rights movement was launched in the early 1970s. At his Belfast Free Presbyterian Church, Paisley preaches anger, hatred and fear, warning his congregation that Catholics will rise up to murder them in their beds if the British ever depart. At the resumption of talks between the British and Irish prime ministers, he called for the formation of a citizens' army to protect Protestants.

For that reason--Reagan favors a peaceful solution to dreary Ulster's endless troubles--Paisley was denied a visa for the United States, where he thinks he is grievously misunderstood.

Paisley's wife, Eileen, and three other Irish Unionists, were permitted to come, and they spoke yesterday at a somber lunch at the National Press Club, which was heavily attended by Bob Jones alumni and other members of what one of them called "the Christian Right."

Mrs. Paisley, who was suffering from a cold, read the speech that her husband wrote. It was a mere hiccup compared with the thunders that shake the Free Presbyterian Church at Sunday night prayer services. To Paisley, the problem in Ulster is the Irish Republican Army, its benighted Irish-American partisans and the communists who arm the gunmen.

Actually, the State Department shares the view that the IRA is working hand-in-glove with other Marxist terrorist organizations. It was a point emphasized by Norah Bradford, widow of the Rev. Robert Bradford, a member of Parliament murdered by the IRA last November. She noted that the Red Brigades, kidnapers of Brig. Gen. James L. Dozier, had asked for support of the IRA.

John Taylor, a member of the European Parliament, said at the luncheon that communists in Eastern Europe support the IRA and that it is linked to the Palestine Liberation Organization.

The fourth visitor, Peter Robinson, a member of Parliament, spoke of the American organization Noraid, which buys "communist weapons to kill British citizens."

What has driven Paisley to moderation--he is now offering a 50-50 power-sharing plan with Catholics below the Cabinet level--and his compatriots to raising the red menace is the specter of unification, a haunt much intensified by William P. Clark, newly designated national security adviser, who declared recently in Dublin that Americans are hoping and praying for unification.

"The southern Irish," said Taylor, when asked about reconciliation, "are a different people." All that north and south have in common is that "we do have to live in the same land mass."

U.S. support for the IRA is almost another red herring. Under the prodding of the "Four Horsemen"--Sen. Daniel Patrick Moynihan (D-N.Y.), Sen. Edward M. Kennedy (D-Mass.), House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and New York's Democratic Gov. Hugh L. Carey--it had largely died out. Passionate sympathy for the hunger strikers caused a resurgence but, since the IRA's return to its wonted barbarities, it has leveled off.

According to Peter Robinson, discrimination disappeared during the the Protestant-Catholic coalition government that lasted less than six months in 1974 in Stormont. Ulster Catholics might dispute that contention. The venture into power-sharing was done in by extremists of both camps--a violent IRA campaign and a general strike by Protestant militants.

Inevitably, the subject of Paisley's ties with Bob Jones University came up in the question period. It was phrased in hostile terms--did Paisley's friendship with "religious bigots" suggest "Unionist support for racial discrimination?"

Taylor denied any knowledge of Bob Jones, which the group will visit during its tour, and declared, "We are opposed to racial discrimination." He spoke of attending a memorial observance for Martin Luther King Jr., whose birthday was celebrated in bitter cold, with bitter words for Reagan, last Friday.

The president is said to be miffed by charges of "racism" in the wake of his reversal of a Nixon administration policy denying tax exemptions to segregationist schools. It was the bureucrats he was after, not the blacks, Reagan insists. But when he found out that blacks had been grossly insulted, he did not back off or apologize. He challenged Congress to undo his work.

The final lament from a Reagan adviser was that "nobody focused on the marketing."

Ulster has had a "marketing" problem for centuries. The Rev. Ian Paisley knows all about it. That's why he's repackaging discrimination.