This is for the Irish primarily: How do you choose your team? Presumably you have no direct connection through geography. Is it a glory hunter thing (as most Irish fans I know support Man Utd or, if they are older, Liverpool)? Also given that you don’t have a direct relationship with your team, could you (do you) switch when the going gets tough?
I have a freind who lives in Northern Ireland and supports Celtic and Liverpool. The Celtic I understand, he is Catholic etc. And I think Liverpool has always ahad some sort of connection to Celtic, at least they both adopted “I’ll Never Walk Alone.” if I recall.
Is there a resource that gives average, minimum and maximum salaries in all leagues? I started thinking about this the other day, does Landon Donovan make more as a sub in the Bundesliga than he did as a star in MLS? And how does Bundesliga pay differ from EPL? Then last night, watching Sheffield United match up well enough with Arsenal, it got me wondering how the folks on Sheffield are paid compared with those on the Arsenal side.
Both Liverpool and Man Utd have historical associations with Irish & Catholic communities. (But I can’t help feel they wouldn’t have the same support from the old country if they weren’t high-flying teams 
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I think some Irish started supporting their club because of the gloryhunter thing (for example, nearly all the Irish Leeds fans I know are around the age of 40), some because their father or other older relative did, some because of the perceived Irishness of the club (this has a lot to do with why Irish people follow Celtic, obviously, but also why people of a certain age follow Arsenal - think Liam Brady and David O’Leary).
Although there are a truly baffling (IMHO) number of “fans” who have never set foot in their favourite club’s hometown let alone ground, I haven’t noticed any tendency for them to switch favourites when the going gets tough.
You’re right there, its more of a gloryhunting association than any community-based affiliation. The religous partitioning of Everton/Liverpool is regularly mis-reported in the press. There actually is no sectarian divide (thank God!), as both clubs basically grew from the same methodist church football team in the nineteenth century. Both clubs have had links with both Glasgow clubs and the Irish community at differing times in their past.