Interesting thread, especially having some posters with family connections to the building of the hydro schemes and other projects.
There has always been a considerable Irish and Northern Irish workforce involved in any type of construction project, going back at least to the "navvies" (navigators) who built the British canal system. Later this term was applied to road-builders. And of course, many Scots, Irish and other nationalities went to the USA, Canada and the Antipodes in search of work.
The tune of Crooked Jack is the same as "Star of the County Down" and "Van Diemen's Land" ( Come all ye gallant poachers): I've no idea which song had the tune first, but no doubt, someone out there will know!