Before looking at the main body of the poem, it's important to put the title context. The poem is set some time in the past. It's not specified when exactly, but it's certainly a time in which folklore, the supernatural, and agriculture labor held greater sway over society. It's also probably set in Northern Ireland. Lammas Day is a festival on the 1st of August to mark the wheat harvest—again linking the poem to agricultural labor. The speaker appears to take on the "hireling"—his new employee—because he needs more help on his farm.
Moving on to the poem itself, the speaker appears to hire his young new employee at a country fair linked to the festival, and, for some reason, gets a highly favorable deal ("he struck so cheap"). Indeed, the speaker is cheered—he has a "light heart"—by this bit of business. Already, the poem is notable as much for the details it leaves out as the ones it includes—why, for example, did the hireling strike such a one-sided deal?
The short phrases are given a kind of light bounciness by the caesurae in lines 1 and 2:
After the fair, I’d still a light heart
and a heavy purse, he struck so cheap.
The enjambment between the two lines contributes to this feeling too. This lightness of touch helps convey how, at first, things seem to go well for the farmer and his new hireling—a little too well perhaps. The hireling brings appears to bring good fortune and has an instinctive relationship with the farmer's cattle (they "dote" on him). Business booms—in fact, it doubles.
It's also worth noting how the past tense affects the speaker's dramatic monologue. The poem already has a sense of foreboding, a sense that all these good things can't last forever. The word "still" makes an important contribution, separating the time when the speaker had a "light heart" from the poem's present, in which the speaker's heart is anything but light.