Irish traditional music sessions are mostly-informal gatherings at which people play Irish traditional music. The Irish language word for "session" is seisiun. This article discusses tune-playing, although "session" can also refer to a singing session or a mixed session (tunes and songs).

In his "Field Guide to the Irish Music Session," Barry Foy defines a session as:

...a gathering of Irish traditional musicians for the purpose of celebrating their common interest in the music by playing it together in a relaxed, informal setting, while in the process generally beefing up the mystical cultural mantra that hums along uninterruptedly beneath all manifestations of Irishness worldwide

The objective in a session is not to provide music for an audience of passive listeners; although the punters (non-playing attendees) often come for the express purpose of listening, the music is most of all for the musicians themselves. "Audience" requests for a particular song or tune of the players can be considered rude. The session is an experience that's shared, not a performance that's bought and sold.

The sessions are a key aspect of traditional music; some say it is the main sphere in which the music is formulated and innovated. Further, the sessions enable less advanced musicians to practice in a group.