DANCING AT LUGHNASAPlay by Brian Friel. The Abbey Theatre production directed by Patrick Mason. At the Plymouth Theatre. 'DANCING at Lughnasa" is a triple accomplishment: a memory play, a family portrait, and a social commentary on its time and place. The time is August 1936, in the home of the Mundy family, two miles outside the village of Ballybeg, County Donegal, Ireland. The details precisely provided in the Plymouth Theatre Playbill indicate the particularities of Brian Friel's latest work to reach Broadway. The Abbey Theatre production is still running in London, where it won the 1991 Laurence Olivier Award for Best Play. "Dancing at Lughnasa" (pronounced LOO-nuh-suh) begins with a tableau vivant, a fleeting impression of the Mundy family as it faces an uncertain world on this apparently uneventful summer morning. The stillness is broken as Michael Murray (Gerard McSorley) steps forward as narrator to begin the reminiscence in which events are unfolded. Among the topics of conversation as the Mundy sisters go about their chores is the forthcoming harvest dance (the lughnasa) and the possibility that, for the first time in many years, at least some of them may attend. Kate, the bossy, ultra-pious eldest sister (Rosaleen Linehan) characteristically puts an end to the discussion with: "The matter is over. I don't want it mentioned again." But the matter keeps recurring. Meanwhile, Mr. Friel relates the central story of sister Chris's (Catherine Byrne) affair with the feckless Gerry (Robert Gwilym), a salesman with no talent for salesmanship. Chris, it turns out, is the narrator's mother. The attendant story of the family and of its eventual disintegration provides the related themes of the poignant comic drama. Friel extends the compass to the larger matter of how the industrial revolution belatedly came to Ballybeg. The sisters engaged in hand-making gloves are being displaced by a recently opened factory. And because enrollment at her school has fallen off, Kate's job as a teacher may be ending. The playwright demonstrates how world affairs can encroach on rural Ireland when Gerry informs Chris that he has signed up to fight for the Spanish Loyalists. "Dancing at Lughnasa" keeps its main focus, however, on the minutiae of domestic life. The Mundy womenfolk natter and gossip, ask riddles, and tune in dance music on their lately acquired radio (which they dub the "Marconi"). The wireless also prompts one of the most rambunctious scenes; the whole household breaks into dance, filling the kitchen with boisterous movement. Whether in such extravagant outbursts or in its typically quiet and tender moments, "Dancing at Lughnasa" is consistently and sometimes exquisitely captivating. The production staged by Patrick Mason is a model of ensemble playing. (The majority of the cast is from the original Dublin production.) Besides those mentioned, the first-rate company includes Brid Brennan as the shy Agnes, Dearbhla Molloy as the mischievous Maggie, and Brid Ni Neachtain as the simple-minded Rose. In addition to the indispensable Mr. McSorley, who must revert on occasion to Michael's childhood, and the aforementioned Mr. Gwilym, the performance is greatly enhanced by Donal Donnelly as the returning missionary whose service in Africa has left him with indelible impressions of tribal rites. "Dancing at Lughnasa" has been handsomely served by Joe Vanek's country-folk costumes and lovely interior-exterior setting and by Trevor Dawson's lighting. Nor should one overlook the zest and vigor with which Terry John Bates has choreographed the occasional dances for Mr. Friel's exhilarating lughnasa celebration.