[WestJet] YYT $484, YHZ $547, YQB $578, YOW $588, or YUL $590 Return to Dublin - May
Depart St. John's (YYT) on May 12th and return from Dublin on June 10th for $484
Depart Halifax (YHZ) on May 25th and return from Dublin on June 3rd for $547
Depart Quebec City (YQB) on May 13th and return from Dublin on June 3rd for $578
Depart Ottawa (YOW) on May 19th and return from Dublin on June 8th for $588
Depart Montreal (YUL) on May 25th and return from Dublin on June 9th for $590
Departure dates seem to start May 12th and end June 3rd depending on your city of departure and length of stay in Ireland.
From what I can tell, June 10th is the last day you can return from Dublin at the above fares.
Fares shown were run through the WestJet website to verify, with one exception. In order, the get the fare from Quebec City, I had to use either Expedia or FlightCentre.
Flights originate from St. John's (YYT) and aside from that city, you'll have to make either one, two or three stopovers, so accounting for that, you'll have to decide if it's worth it to you.
Flights were found using a combination of Kayak +/- 3 days, ITA Matrix, and of course the WestJet website.