In Ulysses, Mr Deasy incorrectly recalls a story about Tiernan O'Rourke and his wife Derval. Derval is abducted by another Irish king, Dermot MacMurrough. MacMurrough somehow reaches Henry II, the Norman King of England, and thus the Norman invasion of Ireland commences from 1169-1172:
A faithless wife first brought the strangers to our shore here, MacMurrough's wife and her leman, O'Rourke, prince of Breffni. A woman too brought Parnell low. Many errors, many failures but not the one sin. I am a struggler now at the end of my days. But I will fight for the right till the end.
If the Normans really were the 'first strangers' to attack Ireland (I don't believe the Romans ever had an inclination to), how much did they know and what sources did they have for such information? Were Norman diplomatic relations with the Irish common, frequent, formal, or at all extensive? Could the Normans have been at all confident in their invasion strategy (outside of support from MacMurrough)?
I can't answer about the Normans and their knowledge of Ireland, but the Normans definitely were not the first strangers (even if we accept that the contemporary residents of Ireland were not once strangers themselves). I'm not sure the quote actually means that—it's just saying that the wife is responsible for "the" strangers, which is this case are the Normans. Maybe it claims something else in the text.
The Normans weren't even the first "Viking" descendants to invade Ireland. Scandinavian raiders and settlers appeared in Ireland before 1000 AD, most notably in and around modern-day Dublin.
Ireland was definitely invaded long before the 12th Century. Vikings had set up a kingdom in Ireland by the 9th Century. Ireland was very much a part of the very complex Norse world stretching from Ireland, and Normandy all the way to Denmark that existed before the Norman conquest of England. For example, in 988 Olaf Tryggvason of Norway married the sister of the King of Dublin. The Irish battle of Clontarf in 1014 involved Norse forces from a wide area, and the Earl of Orkney was killed during it. The Normans belonged to this same sphere. The sister of William the Conquerer's grandfather married the King of England at the time. Norman leaders would have had generations of experience in Irish politics and trade.