In 2252, Finnegan, a flamboyant Irish upperclassman, tormented cadet James T. Kirk in his first year at Starfleet Academy. Looking back from the perspective of years, Kirk described himself as "positively grim" during that era, a fact that delighted jokester Finnegan immensely. Finnegan tormented Cadet Kirk mercilessly as he was an avid practical joker who would put a bowl of cold soup in Kirk's bed, or leave a bucket of water propped over a half-open door.
This character was only mentioned in dialogue.
The android duplicate representing Finnegan was portrayed by actor Bruce Mars.
In the script of "Shore Leave", Finnegan was described as "a big, freckled bully boy."
Finnegan was the only significant character to wear the mid-23rd century Starfleet cadet uniform. Since Finnegan was a cadet in the 2250s, we can assume this uniform Kirk remembered him wearing was from that era. Since the costume was reused, unchanged, on Starfleet extras in late-2260s episodes, it established that cadets of both eras wore the same design of uniform. (TOS: "The Trouble with Tribbles", "Wolf in the Fold")
At one point, Finnegan referred to Kirk as a "plebe". Assuming this usage corresponded to modern naval usage, this would mean Kirk was a first-year cadet at the time Finnegan tormented him. There was a modern US Navy tradition of upperclassmen inflicting various sorts of odious duties on plebes at the United States Naval Academy, but Finnegan's antics seem to go beyond what was currently tolerated.
Finnegan appeared in the comic book stories "Starfleet Academy!" (Star Trek Annual (DC) volume 2 #2), "Old Loyalties" and "Finnegan's Wake!" (DC TOS volume 1 #54-55). The latter two stories depicted him in the 2280s, as a commander serving as an investigator in the Federation Security Legion who was sent to the Enterprise to investigate the attempted murder of Kirk. Although he took the investigation seriously, Finnegan was still a prankster much to the frustration of "Jimmy" Kirk who fell in his own attempts to get even. Finnegan does acknowledge, in private, that Kirk had a genius for commanding crew loyalty.
A family member of Finnegan's made an appearance in the first of Peter David's Starfleet Academy books, remarking after playing a prank on Worf that he came from a long line of Academy pranksters before Worf taught him a lesson.
His mirror universe counterpart (β) was mentioned in the short story "The Greater Good" contained in the anthology Shards and Shadows. While he was an upperclassman, he took great pleasure in bullying Kirk. However, with Simon Van Gelder's assistance, Kirk murdered Finnegan while still attending the Academy. Kirk later commented that "[e]ven death couldn't wipe that stupid grin" off Finnegan's face.