McGreevey was a Starfleet officer in the science division during the Dominion War of the 24th century. He was among the contingent of Federation troops stationed on AR-558 to protect the captured Dominion communication array there.
McGreevey was extremely talkative, and was intensely disliked by his fellow soldier, Vargas, who complained that McGreevey "never shut up". Even so when McGreevey was killed by a Jem'Hadar Vargas was deeply affected by the trauma. McGreevey had been tending Vargas's wound at the time and, as usual, talking. The sudden quiet following his death, and the wide-eyed expression he wore, stunned Vargas and he would not allow anyone to change his wound's dressing following the incident as it reminded him of his dead compatriot. (DS9: "The Siege of AR-558")