Father Mulligan was a priest at St. Mary's Church in the town of Fair Haven, Ireland. The Doctor often assumed the role of Mulligan during his visits to Fair Haven.

When the Doctor complained of not having a very active role, Tom Paris suggested that they "send Father Mulligan on a retreat to a nearby monastery where he takes a vow of silence and never speaks again." The Doctor responded by telling Paris to "Try it, and you'll be saying Hail Mary's till Saint Patrick's Day." (VOY: "Fair Haven")

Later that same year, when the villagers believed the crew to be spirits, Milo and Seamus suggested telling Mulligan about their discoveries of the spirit folk, but Grace told them that she'd seen the Father vanish after giving one of his sermons. (VOY: "Spirit Folk")