As I had occasion to say about another song lyric here the other day: this is poetry, not journalism. Specifically, in this case, it is a clear example of the use of the literary device known as "metonymy" - that is, the indirect indication of a thing - or person - by naming something closely associated with the thing or person rather than naming the thing or person itself. Thus, "the crown" meaning royal authority, "blade" meaning sword, etc. - including "Galway shawl" meaning a woman who wears a Galway shawl.

It's really a pretty simple little song about falling in love, although it is "open-ended" - we don't know if the speaker never saw the girl again or if he came back as fast as he could and they got married straightway.