I think the speaker has significant respect for his father's work. The speaker expresses astonishment and admiration—"by God"—for the skill with which his father, just like his grandfather, managed a spade and enacted his role as a digger of potatoes. The speaker, himself not a digger of potatoes, evidently thinks of men like his father and grandfather as admirable men of a particular group—"men like them"—and wishes that he had sufficient skill and capacity to follow them. However, the speaker recognizes that people contribute different things to the world. While his father and grandfather were able to contribute work to the world via their digging of potatoes, he himself is not able to work in such a physical way. Instead—while he feels that it is a valuable occupation to contribute to the world as a digger of potatoes—the speaker contributes instead through his own preoccupation, that of writing poetry.

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.