census figures (again) - Irish born (not ancestry). Bear in mind 5.5 children per woman 1850, 4 by 1870s, 3 by 1910s
Also bear in mind having an Irish grandparent or even less, doesn't mean you're still holding shamrocks, eating er, colcannon, hating the monarchy or even Catholic. Your 3 other grandparents and 2 parents might have something else to add to your upbringing too, not to mention the country you and your parents have grown up in.
I hope people can see that one can be 75% English as well as 25% Irish to get ancestry/ passport right?
Right? I mean it's not like the Irish community in the States is still majority pureblooded after centuries, let alone the mid-20th. They may well be feasting on Guinness and bedecked in green on St Patrick's Day (as would their Brit counterparts on that one day of the year), but it doesn't mean they're still the Celtic DNA denizen from the 1800s.