Dave Hanson, a few posts above, is correct according to a book on the navigations that I read long ago.
You have to understand the history. Britain is well laced with cuts/canals, which were built long ago from before the start of the industrial revolution, by gangs who lived a life in camps, as did the people who built the American railroads. They were not popular locally, as navigationers were definitely not 'plaster saints.' The term became generic for hard, hardworking, harddrinking men, who owed no fealty to the local citizenry.
In later years the railways of Britain were built by the same system of work, and in those days quite often by Irish labourers, as work was plentiful and Britons could get better-paid and more settled places, hence 'Irish Navvies.'
It must be 50 years since I read the book. I think it was called 'The Navvies.'