''I would have a poet able bodied, fond of talking, a reader of the newspapers, capable of pity and laughter, informed in economics, appreciative of women, involved in personal relationships, actively interested in politics, susceptible to physical impressions.''Louis MacNeice''s prescription is designed to look ordinary, rather than esoteric, but very little poetry can claim to meet these specifications, stringent in their very wideness. MacNeice''s work matches the world he famously described as ''incorrigibly plural.''Michael Longley, himself a distinguished Ulster poet, has written an introductory essay of meticulous advocacy. His wife, the critic Edna Longley, has supplied the apparatus for students and the general reader.