I always thought of this as a quirk of Irish traditional singers, though not every singer and not every song. Those DebC mentions, particularly Frank Harte, I clearly recall doing that. So do several others I've known from Dublin, so I got the feeling it was common in that city. Frank Harte was such a great source & disperser of songs, I'm thinking that this quirk got passed along in a number of places.
My memory doesn't cooeprate in remembering if Clancy & Makem did this occasioanlly or not, but if they did, that'd have spread this practice even further.

Often, the version of a song I sing is strongly modeled on that done by an Irish or Scottish performer, and frequently the way the words are pronounced are lifted entirely from my source. I remember in early days of learning longer songs and ballads, I tried on this quirk, but found I just didn't have the history of it being common in my life, and it never felt natural to me, even after a ballad class with Frank Harte.

Interestingly, I only remember hearing the spoken last line from those who were deeply steeped in promoting traditional song and never from those who merely dipped a toe in that pool occasionally.

Joanne in Cleveland