Ok as a born and rared Dub I will share with all you guys the meaning in this great Dublin ballad
Twang was a type of sticky toffee sold from hand carts in old Dublin

He lay in wait by James's Gate Jame's Gate is a gate of Guinness's Brewery in James's Street Dublin.

Playing "Billy in the Bowl"
"Billy in the Bowl" was a "serial killer operating in around the Stony Batter district he was handicapped and couldn't walk and propelled himself around in a large metal bowl and would lure his victims by asking for directions and pretending he couldn't hear then getting them to bend over where he would use his very powerful arms to strangle and rob them.
The poor oul "Gather em up" "A gather em up"was a person who supplied the College of Surgeons with cadavers for training in anatomy classes the origin of these bodies was often very questionable but would probably have been as a result of grave robbing Oh yes a Midge is a little pond insect and His Mot is his girlfriend

Hope this goes some way to explaining