Mr Gerety (56), a second-generation Irish-American from Brooklyn, was the director of UN refugee operations for Africa's Great Lakes region. His son, also Pierce, worked in Zaire for the Irish agency Concern until last month.
"Pierce was someone who would jump on a plane when there was a problem; he was hands-on and he was highly regarded for his diplomatic skills," recalled Mr Dominic MacSorley, Concern's former director for the Great Lakes.
"He entered the aid business in the 1960s wanting to change the world, and he never lost those lofty ambitions. He remained deeply idealistic."
Mr MacSorley recalled working with Mr Gerety last year in Kisangani in Zaire, when thousands of fleeing refugees emerged starving from the jungles. "Pierce could be very emotional about the suffering he witnessed, even after years of exposure. In Kisangani, I remember him cradling a dying child and demanding `what can we do about this'."