score:2
According to the tourism board of Iceland, both Iceland Air and WOW Air have seasonal (so not during the whole year) direct flights between Dublin and Reykjavik.
A Skyscanner search shows that unfortunately there is no direct flight during the month of December 2015 (or that flights have not been published yet, so that might be worth checking ~3 months before your dates of travel). On WOW Air website, we can see that the airline actually operates direct flights on Fridays and Mondays, in December.
Regarding your initial question about prices, it is unfortunately not predictable and can vary a lot, so you will have to use a booking engine to figure how much it will cost you, depending on your travel dates and when you fly.
Upvote:3
If you are able to enter Iceland, which is part of the Schengen Area, you are also able to enter all other parts of the Schengen Area - unless you are in some very unusual circumstances. There are direct, year-round flights to Iceland from Paris, Munich, Frankfurt, Copenhagen and Amsterdam amongst others. For example, the cheapest flight to Rejkjavik from Dublin on Sat Aug 22nd goes through Copenhagen.