In Calypso Bloom imagines young curates "rinsing empties and old
man in the cellar." Annotators have disagreed about
the meaning of that last phrase. Most likely it refers simply
to employees cleaning the taproom equipment, but some have
inferred that they are swilling the beer left over in
customers' glasses, or collecting it and selling it to new
Gifford glosses "old man" as referring to the remainder in a
drinker's glass. He suggests (I think) that "instead of
throwing it out," the servers may be carrying these leftover
portions down to the privacy of the "cellar" and finishing
them off. Richard Wall's Anglo-Irish Dialect Glossary for
Joyce's Works says that the phrase refers to "beer slops
which are sold to unsuspecting customers." Again, the
subterfuge would presumably be performed in the cellar.
But such suspicious inferences are probably unnecessary. Slote instead quotes from an entry in Partridge's Dictionary of Slang that defines "old man" as "That part of the beer engine in which the surplus beer collects." I do not know what happens to the beer that goes through the slotted grid under the taps, but perhaps Bloom does. If it collects in a receptacle in the cellar, then he is simply imagining, sympathetically, the pub workers' unpleasant task of cleaning out stale slops at the end of the day.