From what you've said, I would say you are 1/4 irish.
Assuming your grandparents (dads side) were english, it kinda cancels some of the Irish in you out. So basically I think of it like this:
(Grandparents of your dad)
100% english
(Your dad)
100% english (as he was born in england)
(Grand parents of your mum)
100% irish
(Your mum)
50% Irish 50% english (as she was born in england)
(You)
75% English, 25% Irish
In honesty, what I just said could be wrong. Not totally clear on the laws regarding where you are born. But I think it has an impact, since if you are born in England, you are legally a British citizen, I think?