I think it's more, if your grandparent was born in Ireland, you are entitled to apply for citizenship, and if your parent was born in Ireland, you are considered a citizen, and just have to verify the documents and get your passport.
I suspect, but amn't sure, that this is more felxible, and I think they include Northern Ireland aswell, i.e. they include "the island of Ireland".
(Which makes sense, the country now known as Ireland didn't exit until 1922, I'm in my 27s and by grandmother was born in the late 1920s. Someone in the 50s could have a grandparent born in what was then the United Kingdom of Great Britain and Ireland.)
