John Ashton, Warren Sherill, Steef Sealy, Brock Benson and Kevin Hart all gave fine individual performances in Conor McPherson's The Seafarer, a play about two drink-sodden Irish brothers, their friends, and the enigmatic figure who shows up for a game of cards. But these performances — the bullying, the silences, the moments of meanness, misery or amnesia — also meshed to create a mesmerizing world in this Ashton Entertainment production, a world murky with booze, despair and even menace, but one that showed a flickering hint of tenderness and light.