Question

If my great-great-great grandfather was Irish, how much am i?

Answer 1

The convention many people use, and some here object to, is that you are 1/2 of what each parent was. Using that, and numbering the great grandparents instead of typing "great" lots and lots of time, we get 3rd GGP 1/1 2nd GGP 1/2 1st GGP 1/4 Grandparent 1/8 Parent 1/16 You 1/32 
You could be more if some of your other ancestors were Irish. The other part of the convention is that you add the common heritages, so if you find out 2 other 3rd GGPs were also full-blooded Irish, you'd be 3/32nds. You could be less if your 3rd GGP was really half Scot and half Irish, but lied about it so no one would know he was partly Protestant. 

Answer 2

You should be if your great great grand mother is you are