The only instance of bowpot in the OED states "1880 Punch 25 Dec. 298/2 Uncle Bowpot, the florist, lives here. Sech a rummy old image he is." That together with the context of its use in this song suggests that it means "face". Bowpot also means flower pot but why Bill should bring a flowerpot before the beak is a mystery.

David Brookes says (i) that Irish navvies were often concentrated at bridge and tunnel workings in order to avoid fighting between them and the English . The lines "The noise of our tools brave boys, does make the valleys ring / We will drive our piles and bore our holes, by one, by toe, by four" are suggestive of that sort of work. (i) Brookes, David The Railway Navvy: That Despicable Race of Men. (Newton Abbot, David & Charles, 1983) p

Navvying was very hard work; even seasoned agricultural labourers (as Irishmen seeking work on the railway mostly were) found that, at first, they could only work part of a day until they had built up the stamina required.

The Illustrated London News, in 1854, described the process whereby 'Hodge' (a name often used to signify the archetypal English agricultural labourer) acquired the necessary strength and stamina.

"So Hodge, deep-chested and broad-backed, discovers by association and comparison that if he can eat as much meat and drink as much beer as the stranger, he can do nearly as much work, so he sacrifices those parish ties so dear to the ignorant and timid peasant, and takes to the shovel and wheelbarrow with full knowledge that he must run where he formerly crawled"

Navvies could often earn twice as much as agricultural labourers. In 1845 harvesters earned 22 pence per day; navvies earned 45 pence for a 9 hour day [tmwbb]. The high wages made it possible to work less than a full day and still earn enough to pay for the rich diet needed to fuel the prodigious amount of work undertaken by navvies. In this way Hodge could gradually build his strength until he could hold his own as an 'excavator' (Apocryphally the navvy diet was said to include 2 pounds¹ of beef and a gallon of beer a day. It was considered manly to consume huge quantities of beef and beer). A party of directors from the London committee of the London & Birmingham railway saw that there were two sorts of navvy on their line. The company minutes of 14th September 1836 note that one group of twenty-six "efficient excavators formed a striking contrast in commitment with another nearby (group) who were the common labourers of the Country and apparently paid by the day from the little energy which they displayed". It may be that the "common labourers of the country" were new to navvying and had yet to build up the strength needed for a full day's work. (Note that the minutes use the word "excavator" rather than "navvy". "Excavator" was used by contractors, railway companies, and polite society who, with typical Victorian fastidiousness deemed "navvy" to be vulgar. A navvy would, of course, call himself a navvy).