Belfast — It will be a great day for the Irish and for Irish-Americans when President Reagan arrives in Ireland Friday for a four-day visit. But the traditional Irish greetings of ''100,000 welcomes'' will be offset by protests against the President's Central American policy and the global buildup of nuclear weapons.
The visit is meant to be partly a sentimental return to the ancestral homeland by an American president with Irish roots. It is also meant to be a vote-catching exercise among Irish-Americans during an election year.
But the volume of protest, relatively small yet not insignificant, gives the visit a hard edge that will keep security chiefs, government ministers, and the world's news media even more on the alert.
Undoubtedly the majority of Irish people will welcome the Reagans warmly, but there will be a sigh of relief if and when the visit ends without incident.
Last Sunday the Irish Campaign Against Reagan's Foreign Policy organized a protest march through Dublin. It included trade unionists, students, women's groups, a couple of vociferous American citizens, and 200 nuns.
The Roman Catholic Bishop of Galway, Dr. Eamonn Casey, has declined an invitation to attend the ceremony at University College, Galway, in which President Reagan will receive an honorary degree Saturday. Dr. Casey, a noted third-world campaigner, is showing his disapproval of US foreign policy. But he is sending a deputy to the ceremony because he does not wish to show discourtesy to the American people.
Significantly, some 200 members of the Irish parliament plan to boycott the Monday session Reagan will address. This will embarrass the Irish government. Prime Minister Garret FitzGerald has pointed out that Irish concern about US policy will be conveyed directly to the President ''without the need for public demonstrations of a kind that might prove counterproductive.''
The main Irish concern is that a hostile reception for Reagan could jeopardize the influx of American tourists and investment on which Ireland depends heavily. Brian McCarthy, president of the Irish Hotels Federation, has cautioned, ''When Ronald Reagan has left Ireland, it is essential that the Irish people will be remembered as having been courteous, dignified, and at all times hospitable.''
Irish hospitality is fully evident. At the luxury hotel Ashford Castle the Reagans will sleep in a custom-made bed which one local described as ''almost as large as the United States.'' White House aides decided the bed was four inches too high, and Irish craftsmen lopped off the offending statistics.
Irish hospitality is likely to be most evident in Ballyporeen, County Tipperary. It was from this agricultural hamlet that the President's great-grandfather, Michael Reagan, emigrated to the US in 1858. Ballyporeen is a sleepy village with 350 inhabitants. It has six bars, four shops, two schools, two new coffee bars, a Roman Catholic church, two priests, and two policemen. Already one of the bars is called ''the Ronald Reagan.''
Ballyporeen has been besieged by tourists and day-trippers. They snap up souvenirs, T-shirts, plates, plaques, and key rings. They are helping to create a legend that will attract even more tourists after the President has gone. On Sunday that legend will be given the stamp of authenticity when Reagan visits the local church and the Ronald Reagan lounge.
At the same time, in Dublin, there will be a silent vigil led by an Ulsterman , Sen. John Robb. Its purpose, he says, ''will be to express, in silence sounding like thunder in the President's ears, the worry, the anxiety, and the fear of the powerless people of the world.''
No one in Ireland has dared to stress publicly that the main purpose of Reagan's visit to Europe is not the photo call at Ballyporeen but the economic summit in London, with the weightier matters of international finance and the repercussions of the war in the Persian Gulf. But after this weekend in the ''old country,'' no one will be allowed to forget that Michael Reagan's great-grandson, who has made his mark in US and world politics, has also made his mark in Ireland.