WHEN AN Irish-American playwright with a tragic soul begins to write comedy, the result is likely to be sentimenality of the worst sort. Ah, Wilderness is that type of play, fare fit for George M. Cohan, perhaps, but hardly the most relevant thing for a post-Beckett audience.

The problems with the play can be surmounted with an underplayed production. O'Neill was at least making an effort in the play to deal comically with his favorite themes-the inadequacy of human communication and understanding, and the failure of language to save doomed people from their own self-destruction. Overplayed, however, the defects in the play, the dated language and Leave It To Beaver quality of most of the jokes become painfully obvious. The Loeb production overplays badly.

The play is a story of growing-up, specifically O'Neill's development into adulthood-remembered from the healthiest light. The father is understanding, rather than a bourgeois drunk, the mother's tender loving care melts in your mouth, and young love triumphs over nastiness, brutality, and even, God forbid, prostitution. It is O'Neill's salute to the Catholic morality in which he was raised, but that morality has all the abiding qualities of an ice cube in August. And the editing in this production only keeps any virtue that is in the text of the play itself from surfacing.

Archie Smith plays the father. Nat Miller, like a suddenly-animated wooden Indian trying to imitate Groucho Marx. Kay Doubleday as the mother would make Mrs. Portnoy cringe, she is so solicitous. Sid Davis, as the uncle of the central character, Richard, should come across as a lovable man condemned by his own weakness from ever obtaining the woman he loves. George Ede makes him the back-slapping traveling salesman of a dozen stale farmer's daughter jokes.

With-Richard Backus's Richard there is another problem. He is good enough for the role, and at times he becomes a troubled adolescent rather than as a fool who is also pretentious. But his good moments are not numerous enough, and, because he has a good deal of talent, his failure in the role becomes sad, and somehow symbolic of the botched job the whole affair is.

The failure of the play is not totally the actor's fault. If Ah, Wilderness is to succeed it must do so because of a director who recognizes the play's defeets and works assiduously to surmount them. Thomas Grucnewald has not only overlooked the play's weaknesses, he has made them shine like a rotten mackerel. The whole produciton becomes a litany of praise for conventional values against the challenge of art and change as envisioned by a failed poet through three manhattans at a Grosse Point cocktail party. The conception of a production is the director's task and, if nostalgia is what Mr. Gruenewald thought the play was about, then he is guilty of mis-reading his text. Done in the way the Loeb has seen fit to do it, Ah, Wilderness is no better as a comedy than Father Knows Best, a vehicle this production unfortunately resembles.

The audience, all well-dressed, all with impeccably-tied ties, laughed uproarously. They were laughing nostalgically, I think, pleased at the pleasant picture of middle-class life the whole thing was. I realized then that the real problem with the play was not why it failed, but why, as we head heedlessly toward apocalypse, it was done at all.