When priests and believers recite the Trinitarian invocation with which Mulligan blesses his breakfast, "In nomine Patris et Filii et Spiritus Sancti" (“In the name of the Father and of the Son and of the Holy Spirit"), they often make the "sign of the cross" with their fingers, either in the air or by touching successive parts of the body: forehead, chest, shoulders, or forehead, lips, chest. (Grammatically, Mulligan's "Spiritus Sancti" should be the genitive "Spiritui Sancti," but Spiritus is heard often in liturgical settings.)
When the priest in Lotus Eaters turns to the congregation and blesses his kneeling parishioners, "All crossed themselves and stood up." In Wandering Rocks Father Conmee "crossed his breast" while reciting the prayers in his breviary. In Telemachus Mulligan "crossed himself piously with his thumbnail at brow and lips and breastbone," his signum crucis in this instance intended to communicate a piece of social information to Stephen and Haines: the man exiting the swimming hole is a priest.