The first stanza begins by describing the poem's setting: the "Bogland" of the title. Interestingly, it prefaces this description (lines 3-4 onward) with a statement (lines 1-2) about what the bogland is not: a flat, grassy expanse stretching clear to the horizon.
"We have no prairies," the speaker says, "To slice a big sun at evening." Context suggests that the speaker, a collective "We," represents the Irish people as a whole. (Ireland is famous for its extensive bogland, and the mention of the "Great Irish Elk" in line 10 further hints that this is a poem about Ireland. Heaney was also well known for writing about his native country.)
The phrase "slice a big sun at evening" evokes a familiar visual image: the setting sun cut into a wedge shape as it sinks below the flat horizon, like a piece of fruit being sliced by a blade. This doesn't happen in Ireland, the speaker notes, because "Everywhere" you look in the Irish landscape, "the eye concedes to / Encroaching horizon." That is, the horizon line isn't flat: it contains hills, mountains, and so on, which seem to lean toward viewers and "Encroach[]" on their space. The viewer's eye has to "concede[]," or acquiesce, to this encroachment; even if it might want a more spacious landscape, there's none available.
The speaker is thus juxtaposing Ireland's landscape with those of other, more wide-open countries. Although flat, grassy plains exist in other parts of the world, including Eurasia (where they're called the "steppes"), the word "prairie" is particularly associated with the Great Plains of North America. Combined with the later reference to "pioneers" (line 23), the word "prairie" suggests that Heaney is most likely contrasting Ireland with the U.S. (He went on to teach in the U.S. the year after Door into the Dark, which contains "Bogland," was published, so perhaps the country was already on his mind.)
This opening stanza also establishes the form the rest of the poem will follow: free-verse quatrains (four-line stanzas) with relatively short lines. Fully half of the poem's lines are enjambed, as lines 1 and 3 are here, smoothing the flow of the language from line to line. This relative smoothness fits a poem about terrain that, as lines 16-19 describe, slips and slides around like "butter."