"Postmodernism" is a difficult term to define. Often it is associated with a disregard for traditional, conventional rules of art. (See, for example, the postmodern architecture of Frank Gehry or the postmodern music of such composers as Philip Glass or John Zorn [see, for some comedy about this matter, the Colbert link below]). Often postmodernism is associated with a tendency to blend or at least juxtapose "high" and "low" art -- elite art and popular art. Often it is associated with deep philosophical skepticism about the possibility of ever finding truth. (The latter reason helps explain why Marxists, who think they know the truth, are often extremely hostile to postmodernism.)
One point very important to note is that the philosoopher most often associated with "postmodernism," even to the point that some consider him its founder, is Jean-Francois Lyotard. Lyotard was deeply distrustful of grand impersonal narratives, rather than being a proponent of them. As the publisher's blurb about Gary K. Browning's book on Lyotard puts it (see link below),
Jean-François Lyotard is generally acknowledged as the theoretical spokesperson for postmodernism. In 1979, his seminal work The Postmodern Condition challenged the presumption and orientation of modern political philosophy. In particular, Lyotard repudiated the notion of grand narratives and promoted a postmodern acceptance of difference and variety and a skepticism towards unifying metatheories [emphasis added].
Seamus Heaney is not a writer who is often associated with postmodernism -- particularly not stylistically. If anything, his poetry represents a resistance to, and reaction against, postmodernist trends.
Good point! :-)
Of course, if you read Lyotard closely, he tends to make grandiose, universal statements on almost every page, despite his overt distrust of such narrative.
In a rather obvious way, Seamus Heaney has no choice about being a participant in a postmodern age. On a simple basis of chronology, he has lived through the late 29th and early 21st centuries in the western world, and thus by definition participates in the postmodern condition in so far as the age is defined as a period. Moreover, his life as an iconically successful poet involves a global media presence, a globalized career (giving readings and doing book signing across the world) and its enabling technologies. Being a Roman Catholic from Northern Ireland means being part of a post-colonial situation almost be accident of birth.
Although his poems use free verse, which is a modernist innovation, his tone and themes are generally traditional and lyrical, rooted in the land and its history. Although there are references to the troubles in his work, his work does not display any of the technical characteristics of postmodernism, and he is generally resistant to ideology, and more concerned with the local and individual human responses to circumstances than with the impersonal grand narratives favored by postmodernism.