Travelling between Dublin and Helsinki can cost a lot of money when prices are ranging around 300 euros. There are even trips which cost close to thousand euros even if it's just a oneway trip. It goes without saying that you'll have a very nice flight with a k.
I've been using Momondo, halvatlennot.fi and ebookers.com to try to find out the cheapest flights. They all seem good sites, but Momondo has a nice graphical output. After you've inputed the date, where you want to travel from and to, Momondo shows you, not just the prices for that day, but also the prizes for every day of the month. This is done with a chart that shows the lowest prize from the first day of the month to the last day. Underneath it you can find the actual planned flights.
Sometimes it's even cheaper if you fly from Dublin to Estonia and cross the Baltic Sea with a boat. Flight between Estonia and Dublin can cost as low as 70 euros. But there's a cutback into it. Path goes via Latvia and Latvia's pitstop takes about 10 hours.