A very American play has found a very American situation in John Gilhooley's "Mummer's End," which the Folger Theater Group presented over the weekend as a first-time-anywhere production. This will be a long-lived, popular comedy-drama.

Uneven in quality and styles, sometimes awkward in its joinings, its characters and conflicts are usually credible. Some of the uneveness lies in Louis W. Scheeder's staging, which tends to use situations rather than accept them, a directorial trick known as milking.

Philadephia's annual New Year's Day Mummers' Parade is the focus for a four-generation family conflict. At 92, tavern-owner Shooter O'Rourke can look back on 86 years of marching up Broad Street and intends to avert what seems the end of his parades.

His son, Spike, and daughter-in-law, Cookie, have embraced his tradition, although, it develops at cost to themselves. Peaches, their daughter, is married to an upwardly mobile lawyer, Gaspari, who longs to rise above this lowbrow tradition. But Gaspari, an Italian descendant now dominated by Irish ties, is drawn in by habit.

On Trixie and Franjo, the Peaches Gaspari daughter and son, future parade action will depend. Independent Trixie mocks a tradition that has no use of female participants, and her introduction of a historian with a tape-recorder spins conflict.

Thus, here is a generational, ethnic, sociological nutshell of modern America, characters and incidents used to a purpose. Some of these situations, however, do come through as contrived.

When Trixie's family meets her highbrow "professor," they are forced, for comic effect, to seem ill-at-ease. But this is their own home - a place where lowbrows would suffer such middlebrow social blocks, at least I am not convinced that Gilhooley's characters would.

Such a scene is at variance with one very well played by Jean Barker and Joseph Sullivan: her Cookie's delusions about their son MIA in Korea, his Spike sensitive and comforting.

A decided Gilhooley strength is his dialogue, which is tense, pungent and valid.

Asked "Are you a masochist," Cookie ponders: "I don't think so . . . Maybe . . . belong to a lot of groups." Recalling their lost son, Spike cries: "He fought for a parallel, a number, 38." On the other hand, Cookie has gotten a laugh by being knowledgeable about blacks who break records; but later she seems unaware that Hank Aaron passed Babe Ruth's home-run record. At the cost of an easy laugh here, we are made to distrust a previous one.

The casting is spendid, with Frederic Warriner's Shooter a rogueish gem. Faced with difficult transitions for Gaspari, John LaGioia manages to make him credible. With very limited definition. Peaches is well served by Marie Wallace.

In two other cases the player have to rise above roles and direction. Anne Stone's interloper is unconvincing through the staging.That very fine, rising young actor, John Gilliss, convinces us about Franjo, overcoming the script's false build-up.

David Chapman's eloquently clutered setting, including a Schmidt's light-up bar sign, contributes as a good designer's shoud, telling us about the rec room's inhabitants.

Performances are Tuesdays through Sundays, with reservations at 546-4000.