Some interesting coverage of cyclist injury hospital presentations in Ireland here:
The reporting includes fairly balanced coverage of the helmet debate, noting the less severe injuries to those who were wearing helmets and also noting the negative impacts of compulsory helmet laws and the possibility that drivers may treat cyclists with less care if they're wearing helmets.
The research only covers patients who made it to hospital, I guess this excludes a number of fatalities and many more less serious incidents. The research indicates that 12 of the patients who made it to hospital died. to put this figure into context as far as I can tell from the Road Safety Authority of Ireland figures on the Central Statistics Office website the number of cyclists killed on Irish roads in this period 2014-2016 was (including provisional figures for 2016) 33 - https://www.cso.ie/px/pxeirestat/Statire/SelectVarVal/Define.asp?maintab.... So my guess is that 21 of these cyclists died before making it to hospital.
I've been unable to find the actual research paper that was presented at the conference.