This Irish Pub on Capitol Hill in Washington DC has been named the "Best Irish Pub" in the U.S. for three years running, at least. No idea what this means, but it is a place where the brogue is heavily heard and the Guinness flows freely from the tap.
The beer menu includes a dozen or so tap beers, including a wide variety of Irish options--the aforementioned Guinness and the same number of bottled options, including a good cider.
The food menu has American pub fare: hamburgers and cheesesteaks and the like, as well as a group of nicer entrees and salads. They have a set of Irish favorites like fish and chips, shepherd's pie, Irish stew, and the like, and their specials may be even more Irish.
Their big claim to fame is an almost constant stream of quality Irish entertainment, late at night. You can't get within blocks of the place on St. Patty's day. But they have the music year round, so if you want an interesting night activity and you're staying at one of the hotels on Capitol Hill, this might be a nice place to be.
Their website is at http://www.dublinerdc.com/.
Note that lunch service is not their strong point. Expect to have a slower lunch if you go here. Vegetarians have a single option.
Also note that there is a second Irish pub next door. They aren't nearly as good as the Dubliner and don't generally have live music.
Union Station Metro, down D street past the Post Office Museum and the Cap City Brewing Company. Then across the street.