I've always wondered about this idea that GH is based on Sierra Nevada Pale Ale - SNPA is a quality brew no doubt, but the Hooker, as far as I'm concerned, is a more complex, flavourful beer - it has a spicy bitterness that's lacking in SNPA a and a whole range of other quite sweet Americans pale ales. The one bottled US beer that I have come across that I found similar to GH is Flying Dog's pale ale but fat chance of getting that in Ireland. They had a couple of six packs in Redmond's a while back but they're long gone.
As for English pale ales that have that same spicy bitterness, I agree with Tube, Dr O'Kells is definitely up there. I'm often disappointed with the bottled English ales - they never seem to live up to the cask-fresh versions (Landlord is just one example) but O'Kells has a great zesty, tangy bite. The bottle also some has very interesting Manx gaelic design. I wonder could a vintner be harassed into importing some?