Having listened to Mulligan's clowning about old Mother Grogan making water, Stephen hypothesizes that she "was, one imagines, a kinswoman of Mary Ann." Mulligan picks up on the allusion and is soon belting out a song that he and Stephen both know:

For old Mary Ann
She doesn’t care a damn,
But, hising up her petticoats . . .

Given the context, the omitted line must have something to do with urination.

JH 2011

Source: xpedition.be.