You're tired. You've had a hard day at the office. You decide to pop into the neighborhood pub, deposit your hindquarters on a barstool, commune with a tall glass of alcoholic beverage, and quietly start feeling sorry for yourself.

Fine, but steer clear of Gallagher's Pub on a Tuesday night.

Tuesday night is Sheepshead Bay night, and although the four men who constitute Sheepshead Bay are softspoken enough in private, when on stage there is one thing they will not countenance - and that's passivity.

Introducing their rendition of "Marching Down Broadway," lead singer and lead talker Dennis Essig carefully instructs his audience to applaud vigorously after the word "hand," to throw kisses after the word "wonderful" and to raise fists high in the air after the word "Guam" (as in "Remember Wake Island and Guam")? And every fist in the place is dutifully raised on cue.

Every fist but one. One conspicuous fellow in a black overcoat, his chin resting on the bar, has the audacity to keep his fist hand in his pocket and his right hand clutching an empty glass.It turns out, however, when owner Conan Gallagher comes around to investigate, that this culprit is more than passive.He is asleep.

The rest of the crowd - a full house on a rainy, stay-at-home sort of night - is not just applauding, throwing kisses and raising its fists, but also is singing merrily along.

One original composition that inspires even greater than average audience participation is Essig's "Left-Handed Blues," a song, he explains, about a "disadvantaged minority."

"Took me 13 years to learn to play guitar," he sings. "Three and a half to learn to tie my shoes. And I'm still singing those left-handed blues."

Those lyrics are indeed autobiographical, according to Bob Ortiz, who with Essig writes most of the group's material. Essig "plays his guitar upside down and backward, which makes for some difficulties," says Ortiz. "I have been trying to talk him out of it."

Essig, 30, like the other members of the group, has a full-time nonmusical job. He teaches English to foreign students. He and Ortiz, who works in the admissions office at Catholic University, met 12 years ago while they were attending a school in upstate New York, and both subsequently transferred to CU.

The group got its name from the section of Brooklyn, N.Y., where Essig and his father used to go fishing. After some college appearances doing lead-ins for Don McLean and others, they sang on an "open mike" night at Gallagher's about two years ago. Conan Gallagher, owner of the Connecticut Avenue pub, says the group took so much time tuning their instruments and otherwise getting prepared, that he almost walked out. But when they finally began preforming, a half-hour late, they were "fantastic," he says, and they have been regulars ever since.

"I was a patron, says Tom Scullen, the third member to join the group. "So one night I asked Bob did they want a bass player." Scullen, who works for the Secretary of the Army, grew up in Silver Spring and learned to play the bass after one was donated five years ago to the church folk group to which he belonged.

The fourth and last member to join was Rob Brager, a friend of Scullen's, who came to Washington from Calgary, Alberta, majored in special education at the University of Maryland, and works at Second Mile House in Hyattsville, which he describes as a "residential crisis center for abused children, throwaways, runaways and what not."

They are about to perform a love song, Essig announces. "This is a song about girl meets boy, boy meets girl, boy takes girl for a ride, girl jumps in tidal basin, girl dries out, boy dries out, girl writes book, boy dries out, girl makes movie, boy dries out."

It is the "Ballad of Wilbur Mills and Fanne Foxe," which includes the refrain. "She was only a stripped at the Silver Slipper, but she had her ways and means . . ."

The audience - particularly one young woman sitting at ringside and shaking a rubber chicken in time to the music - seems to know "The Ballad of Wilbur Mills and Fanne Foxe" by heart.

Ortiz recalls that the group once appeared on television with Fanne Foxe, and while they sang that number, the camera maintained a close vigil on her face looking invain for a response.

On another occasion, they were invited to appear on the short-lived 7:30 Live show. "We were on the night they had a bank robbery in Silver Spring, so they said, 'And now we switch you to . . .' and by the time they got back we were just finishing," Ortiz said.

The performance is peppered with political references and inside - Washington talk. A version of "The Man Who Never Returned" has been reset in the tunnels of Metro - a completed a Metro - and manages to rope in retiring Police Chief Maurice Cullinane and ex-bus mogul O. Roy Chalk.

At most bars where live entertainment is offered, the performers are just another piece of interior decoration to go with the plants and candles. And they get about as much attention.

"We demand people's attention," says Scullen. "We fight for it. We're really pretty nasty."