For a start Forget about C - Most session tunes are in D or G and their associated minors , though A minor does get in quite a bit too.
Harvest Home and Boys of Blue Hill are a regularly paired set of Hornpipes. Speed the Plough, Brighton Camp(The Girl I left Behind Me)
Wister Gallop, Rakes of Mallow, Dingle Regatta, that shold ne a good start.
Regarding Keys - There is a tendency to keep to keys that the ubiquitous D/G Melodeon players can manage though IF there are a preponderence of fiddles A crops up a lot too. And an Oboe and a Flute would be a nice addition to most sessions.