Ryanair is the only operator currently flying from Stansted to Dublin:
http://www.stanstedairport.com/portal/page/STN %5EFlight+Information%5EDestinations+and+Airli nes/
I've flown the route several times and I've nothing but praise for Ryanair. They've got a far better punctuality record than other budget airlines and (even though I've flown with the 'big boys', like Qantas) they're definitely my favourite airline.
Ryanair only get 'bad press' from people who object to avoidable charges. For example, prior to Ryanair introducing checked baggage charges, 75% of Ryanair passengers checked bags in. Now only 45% do, because most people have realised that you don't need loads of luggage for a short break. Those that still check bags in moan like mad about having to pay, despite the fact that they still get a good deal when everything's considered. (Incidentally, on the Stansted-Dublin route it's not uncommon for nobody to take checked baggage).
Ryanair are currently offering �5 each way (with taxes paid by Ryanair) on many flights on that route.
Chris