It is that well-known English word "session", recently transliterated into Irish spelling. A usage I find extremely pretentious when employed by people writing in English, though its use in a Gaelic piece would be perfectly reasonable.

The use of "session" for a gathering of musicians may be quite recent, and may indeed have originated in the States, though I would think in relation to jazz rather than traditional music. The informal gathering of singers and instrumentalists in pubs is quite an old custom in the UK, where social life typically revolved more around such places than it did in Ireland until quite recently.