[6] On 7 December 1922, Hales was killed by anti-Treaty IRA men as he left the Dáil.

Another TD, Pádraic Ó Máille, was also shot and badly wounded in the incident.

In revenge for Hales' killing, four republican leaders (Joe McKelvey, Rory O'Connor, Liam Mellows and Richard Barrett) were executed the following day, 8 December 1922.

[7][8] According to information passed on to playwright Ulick O'Connor, an anti-Treaty IRA volunteer named Owen Donnelly of Glasnevin was responsible for the killing of Hales.

Seán Caffrey, an anti-treaty intelligence officer, told O'Connor that Donnelly had not been ordered to shoot Hales specifically but was following the general order issued by Liam Lynch to shoot TDs or senators if they could.