A session (seisiún in Irish) is an informal gathering of musicians who play tunes usually in a pub. In "open" sessions anyone is free to join in (provided they know the tunes). It's also one of the best ways to hear this music.  To find a session near you, check out thesession.org (it's also a great resource for tunes!)

Open Sessions in NYC

Monday

Landmark Tavern
626 11th Ave at 46th St.  A lively session in an historical landmark tavern, led by Don Meade.
(212) 247-2562Monday 8-11PM

Iona Scottish Session
180 Grand Street, Brooklyn. Is Scottish music your thing? Then this is the session for you.
Mondays 9PM-late

Tuesday

Dempsey’s Session at Sláinte
304 Bowery (between Bowery and Bleeker). Led by John Nevin and friends. Open to musicians of all levels. This is the session I call home.  I'm here every Tuesday. 
Tuesday 8–11PM

The Green Room 
156 E. 23rd Street (btw Lexington & 3rd Aves). A bi-weekly open seisiún.
Every other Tuesday 8-11

Wednesday

Gramercy Ale House
272 Third Ave (btw 21st & 22nd Sts). John Walsh and Denny McCarthy lead this session. 
Wednesday 7PM

Dunne's Pub 
15 Shapham Place, White Plains, NY.  914-421-1451. Fiddle master Brian Conway leads this session in White Plains.  
Wednesday 7PM

Thursday

Mary O's
32 Avenue A (between 2nd & 3rd).  Open to musicians of all levels, but aimed at the learning musician. Lots of fiddlers! Led by Caitlin Warbelow and friends.  I love this session.  You'll find me here on a Thursday night. 
Thursday 8-10PM; after session (leaderless) 10:15PM -11:30PM

Fitzgerald's 1928 Pub
13 Herman Street, Glen Ridge, NJ. A nice session in a pub with great food. Steve Wickens and guests. Near the train station.  Thursday 8-11PM

Saturday

Lillie’s
13 East 17th Street (btw Union Square Park and 5th Ave). Dan Neely runs this session that caters to more advanced players.  Saturday 4PM

Slow Session at NYIC
1040 Jackson Ave, LIC (7 train).  Know only a few tunes? Go here. Friendly and welcoming, and not in pub with observers. Saturdays 1:30-4 PM

Sunday

Craobh Úll Mór CCÉ Seisiún
1040 Jackson Ave, LIC. A friendly open session. The 3rd Sunday of every month 4-7PM

The Hutton
225 Hutton St., Jersey City, NJ 07307.  A friendly open session at a lovely gastropub in Jersey City.  3PM -5:30.

For the most complete line up of traditional music sessions, concerts and festivals, check out Ceol Agus Rince.