Do the Irish celebrate Christmas? If so, how do you say "Merry Christmas" in Irish?
Yes, most Irish people do. The Republic of Ireland is predominantly Catholic (though they may or may not consider themselves religious) and Northern Ireland is a mix of Catholic and Protestant, depending on exactly where in the north one is. (Again, they may or may not consider themselves religious). Either way, as both are predominantly Christian, Christmas is celebrated by most people in both the Republic of Ireland and in Northern Ireland.
Merry Christmas in Irish is "Nollaig shona duit" (to one person) or "Nollaig shona daoibh" (to a group of people).
(mildly edited in a vain attempt to dodge potential political quagmire phrasing with regard to ROI/NI distinctions)
'S an cór ón NYPD 's iad
Ag casadh Galway Bay
'S na cloig ag seinnt amach
Ar bhreith Mac Dé