If you want a flavour of how things were during the 1798 Rising and after it (when more people were killed than in the French Revolution), read Mary Leadbeater's Annals of Ballitore, which you'll find in Google Books.

This is the diary of a Quaker woman living in south Kildare, near Carlow and not so far from Wexford, where the worst atrocities were committed (children hanged for not giving information on their parents, etc).

The Quakers behaved particularly well during '98, giving aid and medication to both sides, and were punished for it by the British.

It's perfectly feasible that a cousin would betray someone *after* he was tried. As becomes clear if you read Annals of Ballitore (and other contemporary reports), the courts martial were fit-ups, and once the army had you, you were liable to be dragged off and hanged.

So for someone to shout "That's him, that's my cousin, he's a United man" as someone's led to the gallows, and so seal his fate, is quite believable.

You might also read Jonah Barrington's Recollections of Dublin in the 18th Centry, also available, not in Google Books, I think, but maybe through UCC (University College Cork), which reports the hanging of John Kelly of Killane, who was so tall that the British soldiers couldn't find a tree high enough to hang him.

Instead, they hanged him from a bridge.

His sister, who lived in the town where this happened, looked out from her house on the town square (Enniscorthy, I think), to see her brother's head being used as a football by the soldiers.