A play about a young man dying of a fatal illness runs the risk of turning mawkish. But Irish playwright Aidan Mathews engages the subject of illness and dying with a refreshing candor and lack of sentiment in Communion, a dark and cerebral comedy of manners that is receiving its American premiere at the Origin Theatre Company. Jordan, whose medical studies were cut short by a brain tumor, has returned to his boyhood home in an upscale Dublin suburb. Lying in a hospital cot in his bedroom, sweet-tempered Jordan is cared for and entertained by his brittle mother, his troubled younger brother Marcus, Marcus