What is the Irish "Irish" sweater then?
Well unfortunately its the same as everywhere else really - college students wear Abercrombie and Fitch, Hollister, and so on; or they might wear non-descript plain hoodies from American Apparel or somewhere like that. Yes, Dublin is the same as any other city in that regard. You really have to go outside the cities to see folk wearing aran sweaters and the like - and for the most part they are farmers so their clothes are dirty and worn out.
In Dublin the only ones wearing flat caps and aran sweaters are tourists thinking they are fitting in!