Thanks, Wert.
Ireland... Much better than southern California. If you've never been to SoCal and you're wondering what it looks like outside of LA and San Diego, then watch an episode of MASH or that scene in Austin Powers II where Austin confronts Mustafa and you'll know exactly what SoCal looks like.
Filming on location is important, but for a fantasy story do you think it's more or less important to try and film on location? Some fantasy/epic medieval movies have done well in choosing filming locations, eg. The Lord of the Rings, Dragonslayer, The 13th Warrior, The Princess Bride. Some have done alright in choosing locations, eg. Excalibur, Ladyhawke. Some locations have not been good at all, eg. Krull, Hawk the Slayer, The Barbarians.
The advancement of incoporating CG seamlessly into scenery has allowed Tolkien's, Lewis', and Rowling's magnum opuses (sheesh, I hope that's a word) to come to the big screen. I did not see HBO's Rome, but I assume the CG effects were excellent.
Never having traveled to Ireland, I'll have to rely on pictures and movies for references... is it all green fields and green hills? Is there any other variation in topograpical features of Ireland? Because I think that the North, Winterfell, and even King's Landing could be represented by Ireland. But I don't envision HBO being able to produce the Dothraki Sea and Vaes Dothrak in Ireland... I assume they'll be going somewhere else to film Dany's scenes.
PS - Thanks Ursa... I was, err, editing my post.