Seamus Heaney's poem "The Haw Lantern," part of a larger collection of short poems published in 1987, is not structured as a sonnet, nor does it exhibit the meter and rhyme schemes we associate with sonnets. A traditional Italian (also, Petrarchan) sonnet almost always contains fourteen lines, divided into three sections, an octave (8 lines, a sestet (6 lines), and the rhyme is usually abba, abba (the octave) and cde cde (the sestet—the sestet rhyme may vary slightly. The English or Shakespearean sonnet is usually divided into four quatrains, with a rhymed couplet and a rhyme scheme of abab, cdcd, efef, and gg. The Spenserian sonnet is essentially a combination of the Petrarchan and Shakespearean sonnets.
As we can see by looking at "The Haw Lantern," Heaney does not use sonnet structure. Many sonnets in English use the most common rhythm in the language, which is iambic (an unstressed and stressed syllable) with five sets of iambs to make pentameter. If we scan "The Haw Lantern" line by line we will find that the meter is irregular and internal and end-line rhyme is nonexistent. Although the poem is replete with vivid imagery and metaphorical language, Heaney chooses function over traditional poetic form. The poem is actually closer to vers libre, free verse, or perhaps blank verse than it is to a highly-structured form like the sonnet.
This highly visual and allusive poem begins with lines that personify the hawthorn bush, one we associate with winter because the hawthorn's bright red berries are like beacons in a drab world:
The wintry haw is burning out of season,
crab of the thorn, a small light for small people,
wanting no more from them but that they keep
the wick of self-respect from dying out,
not having to blind them with illumination.
The hawthorn, which is "burning" in winter with its bright red berry, is shining for "small people," an inherently ambiguous designation that could mean the people are physically small or, more likely, spiritually decayed. Because the hawthorn wants only that "they keep the wick of self-respect" alight, we can infer that the hawthorn understands that these morally-challenged people have just enough moral or spiritual vitality to keep the a greater inspiration barely alive in their souls. The hawthorn's goal is not to inspire them ("illumination") but to encourage them to maintain whatever vestige of self-respect resides within them. Heaney undoubtedly chose the hawthorn as the emblem of illumination because it is traditionally associated with Christ and his suffering (the crown of thorns).
In the next few lines, Heaney expands the theme to include a different kind of lantern, but one that is still looking for something inside mankind:
But sometimes when your breath plumes in the frost
It takes the roaming shape of Diogenes
with his lantern, seeking one just man;
so you end up scrutinized from behind the haw
he holds at eye-level on its twig,
Heaney refers to breath that changes to form the likeness of Diogenes, a Greek philosopher, who searches with his lantern in vain for an honest man. Instead of the hawthorn personified, the searcher is actually one's own breath in a form that allows the searcher to engage in a very personal form of scrutiny—looking from his eyes into the eyes of the person being scrutinized.
Unfortunately, as Heaney's last lines suggest, Diogenes does not find what he is looking for:
and you flinch before its bonded pith and stone,
its blood-prick that you wish would test and clear you,
it pecked-at ripeness that scans you, then moves on
Even though the person being scrutinized wishes the hawthorn would, in effect, test his or her blood to confirm inner self-respect, the berry—depicted realistically as pecked (by birds)—finds nothing it seeks, and Diogenes moves to another person, presumably to search in vain for truth, for self respect, for the "illumination" whose brightness is too bright for mankind.