Conor McPherson, that exquisite playwright whose lost, lonely characters often see life through an alcoholic's glass darkly, has now allowed flickers of optimism to shine upon his sombre, Irish world-picture.

The title of McPherson's compelling play refers to a beautiful eighth-century Anglo-Saxon poem of the same name and whose hero sees himself as an exile, literally and metaphorically at sea.

Similarly, the working-class Irishmen, whom McPherson brings together in The Seafarer one glum Christmas Eve, are possessed by existential sadness and loneliness for which lashings of alcohol offers them balm before the morning after.

A quintet of actors bring these characters to comic/emotional life and achieve, in McPherson's own meticulously-wrought production, the finest ensemble acting in town.

The Seafarer is not spiritually or thematically quite like McPherson's earlier plays. A different, more dramatic engagement takes place, in a basement by the sea in a Dublin suburb. The first act title, The Devil at Binn Eadair, hints at what will happen.

You deduce from Rae Smith's insufficiently squalid design with depressing lino and cheap furniture worn to exhaustion-point that this is poor man's terrain.

On Christmas Eve morning Sharky, a love-lorn fifty-something man who cannot keep a working or a private life together, rouses his recently-blinded, tottering brother Richard, who has slept the night on the floor, while their married, hung-over friend Ivan totters down to search for a relieving drop of the hard stuff.

McPherson makes predictable but effective fun of the symbiotically bound brothers, dutiful Sharky vainly trying to clean up his querulous, demanding, drink-prone brother, played to perfection by Jim Norton. The underlying sense, however, is of the men trying to conceal their depression from themselves and each other. When Mr Lockhart arrives with Richard's friend, Nicky, the play takes a not quite realistic turn.

For Ron Cook's suavely menacing, omniscient Lockhart erupts from Sharky's distant past, bearing a diabolical identity, a charge, a revelation and a financial challenge over poker. McPherson makes Lockhart a profoundly poetical Lucifer figure but there is something melodramatic, archly contrived and unbelievable about his function and identity. An over-extended card-game is made dramatic by Johson's extraordinarily powerful performance.

Ablaze with anger and fear, looking at once haunted and harried, frightened and bemused he emerges from the knife-edge game, metaphorically and actually touched by sunlight.