As I understand it, Casey didn't die in prison; he was released on condition he would emigrate to Australia and lived out his short life in Dublin, in disguise, his health broken by ill-treatment in prison, until he died falling from a cab in Dublin city centre in 1870.
The 1798 Rising was a desperate attempt at regime change through revolution by a group who wanted freedom and equality between "Catholic, Protestant and Dissenter" - dissenters being Presbyterians, Methodists, Quakers, etc, and Protestants being the established Protestant church as founded by Henry VIII of England.
This rising was brutally put down by well-armed troops, after which there was a systematic State slaughter of thousands of United Irishmen and Irishwomen throughout Ireland. In Dublin the bodies of the hanged, shot and piked were piled up on the place ever since called Croppies' Acre, for the tide to take them away. This was before the quays were built, and the Liffey was washed by tides twice a day then; Croppies were the United men, who cropped their hair short; they and their sympathisers wore a sprig of green or wore green clothing which was taken as a reason for execution of anyone so dressed in '98.