"In nomine Patris et Filii et Spiritus Sancti": continuing his unending impersonation of a priest, Mulligan blesses the meal with the Church’s familiar Trinitarian formula, “In the name of the Father and of the Son and of the Holy Spirit.”
The priest's words are often accompanied by touching his finger's ends successively to his forehead (Father), chest (Son), and shoulders (Holy Spirit), or, alternatively, to forehead, lips, and chest. Later in Telemachus Mulligan makes the sign of the Cross in this second way. The Signum Crucis is also sometimes made in the air, facing outward, when the priest blesses something—as Mulligan might do in dishing out the fry, were his hands not full.
Grammatically, "Spiritus Sancti" should be the genitive "Spiritui Sancti," but Spiritus is heard often in liturgical settings.