Do you know any sports shops within ireland that flog american football jersey?

Answers: No, but if you're interested in authentic American football jersey, let me know as I'm a retailer.
Specify squad, player's name and size and I'll go and get you a price on authentic Reebok jerseys & cost to ship to IRL.

I don't know any shops IN Ireland, but you can buy them online at:
http://www.nflshop.com/home/index.jsp probably because ancestors buy them.

E-bay my friend.