I should probably have added a few more details to my original post. Here is a link to a transcription of the song from The Gaelic Songster on the NLS's online archive.

I generally see "Biodh an Deoch Seo 'n Làimh Mo Rùin" translated as "The drink would be in my love's hand".

Like many Gaelic love-songs, it's about how much a woman misses her lover. Unlike many Gaelic love-songs, it's not completely doom-laden. The seafarer in this song is neither in imminent danger of drowning (Coisich a ruin) or being unfaithful (Fear a bhata).

Fhir a chunnaic air an t-sàil' i,
Men that saw her at sea
Beannaich an long bhàn 's a crew.
Bless the white ship and crew.

(Thank you, Google Translate.)