This. The vast majority of people support Celtic in addition to supporting their premier league team.
For example, my school organises a yearly trip to Celtic Park to see a home game and train with the coaches as it's the one football club that won't divide anybody.
Not to mention that Robbie Keane is from Tallaght, a working class area (was in his day, still is but to a lesser extent) in Dublin, as evidenced by his thick Dublin accent. Working class Irish people are also much more likely to support Celtic fervently.
Never been a Celtic fan myself, however. Always seen it as a sign of 'plastic' republicanism/nationalism on the part of southerners.
Always wanted to do a study on the positive and negative effects Celtic and Rangers have had on Irish (and Northern Irish) life over the past few decades.
An interesting thing to note is if you youtube the Love Ulster riots in Dublin a year or two back you'll see a number of the "nationalist youths" are wearing Celtic jerseys.
I've often said if you can solve the Celtic/Rangers rivalry you'll have peace in the North!