I owuld venture that 80% of the Irish and Quebec accordeon music is in the key of "D".
Long list of players of both styles that play in D the majority of the time.
I pretty much agree with what Bohemian said and wanted to add some detail.
-- Most one-row players in Quebec use a D box as their primary box.
-- MOST tunes in the repertoire in Quebec are playable on these D boxes. In other words, the majority of tunes are either in D major or related keys (such as B minor), or in A or G and related keys, playable on the D box by substituting notes.
--So a really expert player will be able to sit in a session of random tunes chosen by fiddlers and be able to play most of the time -- just sitting out for a few oddball tunes in keys that aren't really playable on the D box. That said, in the total repertoire, the number of tunes in G major and related keys is definitely higher, and many of those tunes take a high level of skill to play on a D box.
-- My experience with Irish music is more limited but there have been statistical studies, and I know that G tunes (and related modes) predominate. I once went through the entire Ceol Rince collection (a huge collection of Irish tunes in sheet music) and noted which tunes would be playable on a D box -- it is a very high number, including many tunes in G that lack the C natural! -- but again, many many tunes contain a C natural, or a G#.
It's not just the note substitution that is involved in the skill, by the way, it is also the way that the feel of the instrument changes completely when playing in G-related modes. A-related modes are a bit easier.
A lot of players have a second one-row box in C or, less predominantly, in A. This is mostly for solo play because the players prefer the "plus douce" sound as Mr. Messervier once told me. Other players have a three-row as their second box, usually G/C/F and less predominantly A/D/G. And in some areas the three-row boxes are the players' "first" box.
Alan Bradbury of Cajun Magnolia, based out of RI, plays in D to accommodate the female singers. I saw them a while back and they are excellent. Everything is pretty much in D.