Kirby was an orderly at a Federation battlefield hospital on Ajilon Prime during the 2373 Klingon invasion. It was during this invasion that he recruited Jake Sisko to help him move injured patients in the triage.
The two got along well at first with Kirby reassuring Jake that he too had vomited on his first day. Later, Kirby's light-hearted opinion about death at the hands of a Klingon made Jake, who felt guilty over leaving an injured Doctor Bashir on the battlefield, lose his temper. (DS9: "Nor the Battle to the Strong")