With such an in-depth knowledge of his history you could almost be accused of being a fan! He's certainly lived here several times but I'm fairly sure he got US citizenship soon after he started appearing on TV. His Oxbridge accent was presumably developed while in London as it seems to open you up to a much higher level of Hollywood part than an Irish accent does!
Please let me clarify! I copied all that info after a google search!!!
And he does play on both sides of the street when it suits him doesn't he...."I'm Irish, what, the part is for an Englishman?? Wait, I hate the Fenian bastards! Did I mention I'm English, God Save the Queen, what time is tea?"