In this poem, an adult world is seen through a child's perspective, full of the possibilities of imagination.
The speaker travels with a group, noted by the plural first person used throughout the poem. One can envision this little group of children scampering up a slope when suddenly, they are all eye-level with a technological wonder: the telegraph. This technology has faded from our modern society, but a quick image search will provide the structure needed to understand the visual of the "white cups" along the tops of these poles.
In an interview, Heaney noted that in his childhood, around the turn of the twentieth century, his house sat alongside a railway. Along the railway, these huge telegraph poles and wires were distributed to mark the landscape. And along those telegraph wires, Heaney often found raindrops.
As a child, he thus concluded that the messages of telegrams were carried through these little raindrops. This is echoed by the speaker in the poem:
We thought words travelled the wires
In the shiny pouches of raindrops
The theme of childhood innocence is evident in this line, full of the possibilities of understanding a complex world and less worried about what is and isn't possible. Instead, the children understand that
We were small and thought we knew nothing
So they explore, finding the curves of the "sizzling wires" reminiscent of "lovely freehand." Everywhere they look, the children find ways to bring meaning to their world and to appreciate the beauty of it. The tone is bright as the children consider that those "shiny" raindrops are
seeded full with the light
of the sky.
Repeated images of light convey the children's sense of hope as they navigate a world of railways and telegraph lines. This sense of hope supports a theme of childhood innocence; they don't understand the complexities of this world, but they do understand its beauty—and for children, that is enough.