A gunsmith. L&M suggest either Edmund or George, although William sounds more likely.
The Truelock family were Irish gunsmiths. I haven't spent much time looking into them, but quickly found this story about the family home in the 18th century. In part it says:
"In the 18th century affairs of honour were settled among the gentlemen of the day wielding pistols at dawn. In 1777, at a meeting in Clonmel, a committee of Irish aristocrats drew up a duelling code known as the Irish Code Duello, which was then adopted as the official rulebook of duelling throughout the first world.
"So popular was duelling at that time – with Phoenix Park a favourite killing field – there were 19 companies in Dublin who produced pistols. One such enterprise was run by the Truelock family – immortalised by Joyce in Ulysses and also by Jonathan Swift in The Art of Punning.
"Number 64 Carysfort Avenue, a double-fronted Georgian property dating from 1824, was home to gunsmith William Marshall Truelock, who switched career once men of honour decided to settle their differences by means other than firearms."
So William is a traditional name in this family. Feel free to investigate if you feel like it.
Chart showing the number of references in each month of the diary’s entries.