A plaque was unveiled at a ceremony attended by Sargent Gaynor’s family as well as serving and retired Irish army personnel and politicians.
Sargent Gaynor, who was 27 at the time, was serving the UN as part of Ireland first major overseas peacekeeping commitment.
His platoon of 11 was protecting the Luweyeye River Bridge in Katanga province, where a secessionist movement, backed by Belgium, France and Britain was attempting to sever the mineral rich province from the newly independent Congo.
They were set upon by over 100 Baluba tribesmen in a surprise attack and despite a valiant defence were overwhelmed and nine Irish troops were killed including the commanding officer Lieutenant Kevin Gleeson.
Over two dozen Baluba attackers were also killed.
The ambush remains the single worst tragedy involving Irish soldiers serving abroad.