Oh man! Have I chased down a number of rabbit trails about this seemingly-simple song.
Here’s what seems to have happened: The inestimable Wikipedia says, “Similarly to many folk songs, the origins of this song are obscure.” To put it mildly! However, it seems to be the case that sometime in early Irish/Celtic history (and who knows what time period this actually was) a tradition developed of “hauling the bride home,” which took place a month after the original wedding and consisted of the new bride’s being “hauled” to her now-husband’s house. It isn’t clear to me where she’d stay for that month–at her father’s house, one would guess, but who knows? It’s important to note that the original folk song (if there is indeed such a thing) consisted only of the chorus.
Anyway, here’s how (we think) it went, again from Wikipedia:
The “Hauling home” was bringing home the bride to her husband’s house after marriage. It was usually a month or so after the wedding, and was celebrated as an occasion next only in importance to the wedding itself. The bridegroom brought home his bride at the head of a triumphal procession—all on cars or on horseback. I well remember one where the bride rode on a pillion behind her husband. As they enter the house the bridegroom is supposed to speak or sing:
“Oro, sé do bheatha a bhaile, is fearr liom tu ná céad bo bainne
Oro, sé do bheatha a bhaile, thá tu maith le rátha.“
Oro, welcome home, I would rather have you than a hundred milch cows:
Oro, welcome home, ’tis you are happy with prosperity [in store for you].”
The piper, seated outside the house at the arrival of the party, playing hard [i.e. with great spirit]: nearly all who were at the wedding a month previous being in the procession. Oh, for the good old times!