When you speak of the Protestant "Irish," you must be referring to the Scots-Irish. The Irish of the Northern states, particularly the Northeast, like the Kennedy clan, are immigrants from Ireland, and are, of course, Catholic.
The Scots-Irish from Ulster, or Northern Ireland, a very different story. Protestants to the man, making up a large part of the south, Bible belt Protestants.
the Italians and Greeks, all Catholic
Wouldn't the the Greeks be Greek Orthodox?
There are TWO major waves of Irish immigration. The first is in the Colonial period ~ and the second is from the 1830s to the 1860s.
During the colonial period the Irish who arrived in America had no friendly Catholic church to greet them. Instead they had Scandinavian Lutherans, English churches (Anglican and Puritan, etc.) and the Scotts ~ who were something, or nothing, but fur shur were not Catholic.
Not that the English had banned Catholics from America, but the original Treaty of London (1604) put the core area(Virginia) off limit to Catholics. That was on the orders of the King of Spain who was doing a cramdown on his enemies and his friends!
As a consequence there were no religious in the area and without them you don't even get an underground Catholic church movement ~ what you get are Protestants, and that's where the majority of America's Irish descend from ~ that first wave of immigrants back in the colonial period.
The Pope, who claims primacy over all Catholic bishops, recognizes the baptism of many Protestant bodies ~ my own included ~ and we recognize his as well and boy was there a fight over that one!
Now, regarding the Kennedy's, there are probably more Kennedys whose lineage dates from the Colonial period than from the Famine period. They were here longer, bred frequently, had enormous families, and spread out all over the country. The Murphys are even more numerous and have, to their credit THREE GREAT AMERICAN HEROES ~ Timothy Murphy, the most renowned enlisted man in the American Army in the Revolution, and Audie Murphy, the most decorated soldier in the Armed forces of the United States in WWII. More recently another Murphy was awarded the Medal of Honor.
I know the first two are Protestant ~ this latest guy may be Catholic. But they're all Murphy, and that's what counts!