In the first stanza, the speaker of the poem sets the stage - he is sitting and writing near his window, with a pen in his hand, where the pen feels snug and comfortable.
In the second stanza, we learn that while he does this, his father is "rasping," or breathing hard outside as he digs with a shovel in the "gravelly ground." His son looks down ...
And in the third stanza, he notices his father "straining ... among the flowerbeds," but when when his father stands up after bending down, the son is transported to 20 years before, where he sees his father digging, tirelessly, for potatoes.
In the fourth stanza, he pictures his father with the drill, comfortably and confidently performing his job, the shovel snug to his body, as he both digs up potatoes and scatters new ones. The son grew to love the feeling of the cool, hard potatoes in his hands.
In the fifth stanza, he moves from talking about his father to recalling his grandfather. Both men were very skilled with their shovels.
The sixth stanza focuses on his grandfather, who did not dig potatoes but turf. He, too, was very good at his job - able to "cut more turf in a day/Than any other man on Toner's bog." The speakers remembers a time he brought his grandfather a milk to drink, and his grandfather stood up, drank it quickly, and got right back to work, barely breaking his rhythm of his heaving and digging.
The seventh stanza returns to the present, though the smells of potato mold, the sounds of the soggy peat, and the feel of the "cuts ... through living roots" hang in the air and "awaken" in the speaker's head. He knows, though, he has no talent or desire to follow in the footsteps of his father and grandfather in digging with a shovel. Besides, it's a different time - an industrial time - and fewer find their lievelihood in manual labor.
Instead, in the eighth stanza, the speaker says he will "dig" with his pen, as he thinks about his heritage and writes down the stories of his childhood.