When we hear that Mulligan "crossed himself piously with his thumbnail at brow and lips and breastbone," we may suspect that he is indulging in one more mocking imitation of Christian holy men. But in this instance his Trinitarian gesture also is intended to communicate a piece of social information to Stephen and Haines: the man exiting the swimming hole is a priest.
The narrative confirms this five lines from the end of the episode, by referring to "The priest's grey nimbus." Gifford suggests that the man’s “garland of grey hair” just before Mulligan’s gesture indicates that he has a tonsure, but the phrase might just as easily describe the ring of ear-level hair that many balding men acquire naturally. In Proteus Stephen recalls the "garland of grey hair" as he thinks of the (supposedly) bald Joachim Abbas.