If you ask in City Tailors, 57 Mary st, D/1 (01) 8734268 they might be able to do something with it. New York Dry Cleaners in Abbey St. might give you a recommendation also.
I think I saw a recommendation on AAM once upon a time for somewhere near to Grafton St. but can't recall the name. Perhaps someone else will remember it.
A small trick you could try yourself is to get a small piece of material something similar in colour/weight to the shirt. Use a small piece of [broken link removed] and as neatly as possible iron the patch onto the inside of the sleeve with the Wundaweb. Don't overlap the tear just draw it closely together. Hope this makes sense. If it doesn't work out there is nothing lost as you can just take the patch/wundaweb off.
You can buy the wundaweb in Hickeys.