There's a song from the fifties, written by Sean McBride, which may have given Dylan the tune for I Pity The Poor Immigrant (or perhaps even I Dreamed I Saw St. Augustine). I'm not sure whether anyone else has noticed, or whether it just sounds like it to me. I suppose he would have heard it in Britain or from the Clancy's.
The song is called The Homes of Donegal.
You can hear the melody that Dylan may have taken particularly in the instrumental part before the singing starts. It's not an obvious lift, though, and perhaps the similarity is just accident.
Here it is sung by Johnny McEvoy so you can make up your own mind.http://www.youtube.com/watch?v=kBIBQk99ieE
That melody is from long before the '50s, and Dylan used it before "Poor Immigrant" in the "Ballad of Donald White." Dylan's source was the "Ballad of Peter Emberly" sung by Bonnie Dobson. But this melody is the tune of so many songs, it's outrageous, "Tramps & Hawkers," and "Lakes of Pontchartrain" among them. On Shane MacGowan's first album with the Popes, there's several songs that share this melody, and it is also the basis for "Huck's Tune."