"The 1798 rebellion was possibly the most concentrated outbreak of violence in Irish history, and resulted in thousands of deaths over the course of three months. Contemporary estimates put the death toll from 20,000 (Dublin Castle) to as many as 50,000 of which 2,000 were military and 1,000 loyalist civilians. Some modern research argues that these figures may be too high. Firstly, a list of British soldiers killed, compiled for a fund to aid the families of dead soldiers, listed just 530 names. Secondly, professor Louis Cullen, through an examination of depletion of the population in County Wexford between 1798 and 1820, put the fatalities in that county due to the rebellion at 6,000. Historian Thomas Bartlett therefore argues, "a death toll of 10,000 for the entire island would seem to be in order". Other modern historians believe that the death toll may be even higher than contemporary estimates suggest as the widespread fear of repression among relatives of slain rebels led to mass concealment of casualties."
As usual: no-one really knows the figures though we can probably all agree they were pretty appalling. Bringing in the French probably didn't help.