The best answer to this question is that Heaney uses middle diction in this poem, as he does in the majority of his poems. The word choice employed in this poem is definitely not formal, as the words are, on the whole, common and everyday words, easily understood. However, the words are definitely not informal: there are no colloquialisms or slang employed. Note for example the following quote from the poem:
I can see her drowned
body in the bog,
the weighing stone,
the floating rods and boughs.
These words are easily understood, and there is no attempt to use elevated vocabulary that would be seen in poems featuring poetic diction. Correct language is maintained, and as a result, the diction in this poem is middle diction. Note, however, that even though middle diction is used, this in no way undermines the impact of the poem, and in many ways only enhances it. The relative simplicity of the diction used helps to make Heaney's message about punishment in general, whether it is in its ancient form or the more modern day form he goes on to talk about, loud and clear. The simplicity of the language elucidates the theme of the poem, which is Heaney's own reluctant understanding of the need to punish and the tribal revenge that occurs all over the world, not just in his native Ireland.