Nanny O'Brien's Irish Pub - 3319 Connecticut Ave., N.W., Wash., DC 20008 -Tel: (202) 686-9189 // sessions usually Mon, but you could check
There is a weekly session in Baltimore on Thursdays at J. Patrick's Pub at 1371 Andre St. in the Fort McHenry area. On the way to Fort McHenry (the intersection before the fort) is Andrea street, turn left, and go 1 block. There is live music a J Patrick's every Friday, Saturday and Sunday, if you just wish to listen.
new Sunday session 4:30 to 7:30 at the newest Irish pub in town, Paddy Mac's, 8241 Georgia Ave (upstairs), in Silver Spring. The SS metro stop is just a few block away, and there's plenty of parking around as well.
Some times The Cat's Eye, in Fells point (Baltimore) has good Irish music (for listening) as well, but it its hit-and-miss

There is also a session on Tuesday nights at Mick O'Shea's Pub, 328 North Charles St. in Baltimore.