The missing lines obviously go at the end of the third verse. I recall finding them in an old songbook in my aunt's house in County Down but don't remember what it was called. The full verse is therefore as follows:

He swallowed tripe and lard by the yard, we got scared
We thought it would go hard when the waiter brought the bill
We told him to give o'er, but he swore he could lower
Twice as much again and more before he had his fill
He nearly supped a trough full of broth, says McGrath
He'll devour the tablecloth if we don't hold him in
He ate a dozen fowl, half a cow, made a vow
He was just as hungry now as when he did begin