In the basement bar-lounge of a Capitol Hill pub, some energetic and dedicated young people in love with the theater are showing what can be done with imagination and talent - and no stage or money.

The prism Theatre Ensemble is offering the American premiere of "One Evening Gleam," a single-act play by George Fitzmaurice, an Irish play-wright who wrote for the Abbey Theater in the 1920s and then became an eccentric recluse, speaking to callers only through the mail slot in his door.

The Prism Ensemble's production may not be a historic theatrical event, but it is an experience in theater-going: The improvised state is down a dark long stairway in the basement lounge of the Irish Times pub at 14 F St. NW. Getting there is like finding the light at the bottom of the stairs.

And there is some light on the makeshift stage. The four performers who are the core of the non-profit Prism Theater Ensemble have verve, ingenuity and acting polish. Such groups are an important part of the theatrical life and variety of a large city.

Matthew Coughlin, who directed "One Evening Gleam," is artistic director of the ensemble. He and the others support their theater habit by working part-time at other jobs.

Coughlin, who studied theater in college on his way to a doctorate, teaches speech and theater at a local commercial school. Mark Brutsche once worked as a termite exterminator. Maureen McGinnis takes temporary jobs. Chrissie Hines handles the ensemble's business affairs.

"One Evening Gleam" is set in Dublin in the 1940s. Hines, Brutsche and McGinnis give a lively reading of lines rich in humor and do not miss the underlying sweet Irish sadness. There are some nice production touches - the saucer atop teapot, the improvised unbrella dance, the doling out of soda biscuits.

The Prism Ensemble has been in existence since February, 1978, and still is looking for a home of its own. Coughlin promises the group will stage French farces by Courteline and Feydeau and another original comedy in the vein of "Feets," staged last fall.

Performances of "One Evening Gleam" begin at 8:30 p.m. Fridays and saturdays through July. The Irish Times is near the Union Station Metro stop.