First appearing in X-Men #28 (January 1967), Irish mutant, former Interpol agent and NYPD officer, Sean “Banshee” Cassidy possesses a "sonic scream," capable of harming an enemies’ auditory systems and causing physical vibrations. Banshee was a decade older than most of the X-Men and he had a relatively short tenure as a full-time member of the X-Men.
Following the death of his wife Maeve in an IRA bombing, Cassidy lashed out at his cousin Tom, permanently injuring him, and before he could be told of his own daughter’s birth. Tom then raised Theresa by himself. Banshee then left Interpol and unwittingly joined the villainous organization, Factor Three. Cassidy was later shocked to learn of Factor Three’s true nature, but he was forced to obey them when they placed an explosive headband on his brow. He performed various criminal missions on behalf of the organization, during which he encountered the original X-Men. Professor Xavier was able to mentally deactivate the headband and Banshee helped the team defeat Factor Three.
Sometime later, Banshee was recruited as part of the “Second Genesis” group of X-Men to rescue the original team from the living island Krakoa. He remained with the X-Men for several key missions, before falling in love with Dr. Moira MacTaggart and losing the use of his powers when his vocal cords were severely damaged in battle with the terrorist Moses Magnum. Following this accident, Sean Cassidy learned of the existence of his daughter.
Banshee's powers gradually returned as he healed, and he remained an occasional ally of the X-Men, even being instrumental in reorganizing the team after the “Inferno” event.
Alas. in current continuity, both Moira MacTaggart and Sean Cassidy have been killed in action.