Thoughts on the different verses mentioned here. I reckon that the 42nd sailing down the Broomielaw, a stretch of the river Clyde in Glasgow, is likely to refer to a troop ship heading for some theatre of war. However the tattie howkers would have been Irish seasonal workers returning to Ireland after the season,on the regular Glasgow to Belfast ferry which berthed at the Broomielaw. I am only guessing at the cotton spinners verse but they may have been going to Belfast too, to look for employment in the Irish linen industry, just a guess.
As a post script, there is a rocky island in the Firth of Clyde where traditionally they quarried the stones for curling, called Ailsa Craig, and in the days of the regular Glasgow/Belfast ferry service it got the nick name of Paddy's Milestone. I think it may have marked the halfway mark on the voyage or thereabouts.