If the ale is good what matter the size of the vessel?

OED says merely: 'A bowl or dish for containing liquor, properly, one made of wood.' It goes on to expand slightly on the definition, mentioning "any material" and "made of staves", but I think that there is no specific quantity involved. Empty one, start on the next, I says.

BTW, I make the song Scottish, not Irish and in the verse:

There's soldiers and there's sailors and glaziers and all
There's doctors and there's ministers and them that live by law
And our friends in Sooth America, though them we never saw
But we can they wear the work of the weavers

That should be ken, not can.