Maggie O'Halloran was a character in the Fair Haven holoprogram on board the USS Voyager. In Fair Haven, Maggie operated a vegetable stall. She was engaged at one point to a pig farmer named Ray Ewan, but the engagement was broken off after Fair Haven was repaired following a neutron storm. She also had a sister who had wooden teeth. (VOY: "Fair Haven", "Spirit Folk")

During one visit to the program, Ensign Harry Kim romanced Maggie. Tom Paris decided to play a trick on Kim and changed Maggie into a cow just as she and he were about to kiss. Unbeknownst to Paris, two of the other holograms, Seamus and Milo, were watching, and, when they saw the transformation, were convinced that Paris and Kim were evil spirits. She was restored to her original state by The Doctor after Seamus and Milo brought her to the church during Mass. (VOY: "Spirit Folk")

The Voyager crew later discovered that due to a holodeck malfunction, the holograms in Fair Haven had their perceptual filters go off-line, causing them to become aware of happenings outside the scope of the program (such as when Paris turned Maggie into a cow). The suspicious holograms captured Paris, Kim, and The Doctor. After Captain Janeway explained the situation to Michael Sullivan, one of the characters in the program, he calmed the people down and the situation was resolved. (VOY: "Spirit Folk")

Maggie O'Halloran was played by actress Henriette Ivanans.
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