Well you are in the right place for nightlife and shopping, and for days out get the DART, the station Tara St, or Pearse St, is about 5/10 mins walk from Temple Bar - southside to see Bono's house, just get off at Dalkey or Killiney and get the directions from google Earth, he lives on Vico Road, at the South end of it, Howth is northside on the DART, make sure you get the Howth train, not the Malahide one. 
To be honest i'd leave Cork or any sightseeing down the country until another time, you'd be better doing that in the spring/summer and taking a car across on the carferry. If you are only here for four days, you'll easily pass them in Dublin.
http://homepage.eircom.net/~fluppet/DalkeyWalks/5.html
http://www.tripadvisor.ie/ShowTopic-...ty_Dublin.html
You have a good chance of seeing Larry Mullen out and about in Howth as well, he drinks there regularly:
http://www.independent.ie/entertainm...s-1782416.html

To be honest i'd leave Cork or any sightseeing down the country until another time, you'd be better doing that in the spring/summer and taking a car across on the carferry. If you are only here for four days, you'll easily pass them in Dublin.
http://homepage.eircom.net/~fluppet/DalkeyWalks/5.html
http://www.tripadvisor.ie/ShowTopic-...ty_Dublin.html
You have a good chance of seeing Larry Mullen out and about in Howth as well, he drinks there regularly:
http://www.independent.ie/entertainm...s-1782416.html
Is a lovely walk, nice and scenic
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