He was one of eight men killed by Free State forces in the Ballyseedy Massacre, a defining event in Irish history.
Shortly after, on 6 March, five Free State soldiers were killed by a booby-trapped bomb at Baranarigh Wood, Knocknagoshel.
On the morning of 7 March, O'Shea, Fuller, Shanahan, Tuomey and five other prisoners from Ballymullen Barracks - all selected by Neligan - were taken in a lorry to Ballyseedy Cross.
There, they were secured to each other by legs and hands in a circle round a landmine whose construction was supervised by two senior Dublin Guard officers.
According to the sole survivor, Stephen Fuller - his lieutenant - O'Shea uttered the group's final words "goodbye lads" before the mine was detonated remotely at some time past 3 a.m.
The event was witnessed by a local, Rita O'Donnell, who also saw the uncleared remains later that morning, spread about the greenery.