OK, wikipedia, gimme the tragedy:
On 30 June 1980 during a concert in the Cork Opera House Luke Kelly collapsed on the stage. He had already suffered for some time from headaches and forgetfulness, which however had been ascribed to his alcohol consumption. He was diagnosed with a brain tumor.
The subject of the song, btw?:
[Kelly] was dragged from his bed and hanged by British soldiers who decapitated his corpse and kicked his head through the streets shortly after the fall of Wexford on 21 June 1798.