Sinead Kerr was the younger sister of John Kerr, an officer assigned to USS Enterprise while Christopher Pike was still in command. After John's death during a Klingon conflict, she decided to join Starfleet.
Through her brother, Kerr had known both Montgomery Scott and Spock. Kerr trained as an exobiologist, but had transferred to the Command division and, by 2268, was a lieutenant and acting executive officer of the newly-commissioned Hermes-class USS Adventure. (Cloak and Dagger)