First of all, you should develop the habit of referring to the voice in the poem as the speaker, not the poet himself (or herself). Poems are often autobiographical or semi-autobiographical, but not always.
The speaker in this poem has great admiration and pride for the physical labor his father does. The poem's diction and imagery emphasize the efficiency and prowess the father demonstrates when he is planting potatoes.
The speaker also has great admiration for the work his grandfather, his father's father, did in cutting turf. In Ireland and elsewhere, peat turf was cut to burn as fuel to warm homes. He boasts that his grandfather cut more turf than any other man digging in a particular peat bog. He recalls a specific memory of his grandfather barely taking a break when the speaker brought him a bottle of milk.
Both the speaker's father and grandfather are favroably described as machines—efficient and tireless.
The speaker is careful to characterize his decision to be a writer instead of a laborer as a different, not better, choice.