The poem opens on a deliberately ambiguous note: people described only as "they" await the speaker "somewhere beyond Eden Rock." Eden Rock isn't a real place, but it gently sets up the dreamlike quality of what is to follow, drawing in the reader's curiosity. "Eden" alludes to the biblical Garden of Eden, subtly suggesting that whoever "they" are, they aren't in this mortal world anymore.
The colon at the end of the line indicates that what follows will to some degree be about clarifying exactly who "they" are. Lines 2 to 4 are then devoted to the speaker's father. More specifically, they are devoted to reconstructing what looks be some kind of memory about him. These lines utilize many caesuras to add detail to this description, evoking a sense of care and attentiveness as the speaker goes about building an image of his father.
The father is presented as relatively young (he is 25). The specific age is a strong clue that this is not a real encounter, but some kind of dream, memory, or combination of the two. It means that there is a tension between the measured, adult-sounding tone of the speaker and the age of his father, introducing a note of mystery to the poem (because if the speaker's father were really this young, the speaker wouldn't be old enough to be reciting this poem!).
Indeed, the details about the father support the idea that he is somehow preserved in a particular moment in time. He is wearing the "same suit"—perhaps the "same" one as in the speaker's childhood memories—and his little dog is "still two years old." This "still" suggests that normally this would not be the case—probably because both the father and his dog are long dead.
The "Genuine Irish Tweed" is a very specific type of material as well, adding further nuance to this image of the speaker's father. There's a subtle sense of pride in the fact that the suit is made from a "Genuine" material, perhaps suggesting that this family is of only modest means.
In the stanza's second two lines, gentle /t/ consonance evokes the trembling movement of the dog, but also the way that the image itself is built on the shaky foundations of the imagination:
Of Genuine Irish Tweed, his terrier Jack
Still two years old and trembling at his feet.