Cholera, mostly, but also violence. It says they were turned away from getting help because the locals feared contagion, so perhaps that's some of the violence. They also made practically no money but had a generous whiskey allotment, which would likely contribute to violence. They were working on Duffy's Cut for some time, and the mass grave has been described as a garbage dump for bodies, not like they were lined up and shot or something, not that it makes it any better. https://www.smithsonianmag.com/history/irelands-forgotten-sons-recovered-two-centuries-later-9194680/