There’s been a pretty good fare between Toronto YYZ to Dublin DUB floating around for awhile on Air Canada (Rouge). That deal has gotten even better though! Air Canada is having a holiday promotion for discount codes and free flights, called Gift Massive. Play the game until’ you get a 20% discount code (they’re pretty common, so 2-3 tries tops and you’ll have a 20% off code). Note the 20% off code applies only to the base fare, and you have to fly between January 7 and June 15, 2015, and book by December 30, 2014.
You can play once per day with every unique email address you might have.
Using this 20% promotion code, you can get this Dublin deal for as low as $528CAD/$455USD. There is wide availability throughout the year. I suggest using ITA Matrix to find the availability, and then using your 20% discount code booking on Air Canada directly.