Would not a better POD for the Irish be that Henry II never accepts the Earl of Leinster's position, deciding that his resources are better spent elsewhere or something (or perhaps the Earl actually fails to catch him after spending something like two months chasing him across half of western Europe), and Henry then punishes Strongbow for his involvement, necessitating a pull-out, or at least meaning that when Strongbow's descendants are Gaelicised, they have all links with England cut and the Kings of England see no just cause to declare Ireland theirs? Seems to me that the English are always going to keep coming back unless their claim to Ireland never materialises in the first place...