The Golden Eagle Trust has described the poisoning of two generations of the same red kite family as "totally devastating".
A dead juvenile bird found in Wicklow town in January was the offspring of an adult female poisoned 13 months earlier near Brittas Bay.
Tests have confirmed that the juvenile, identified by its tags as Blue White 21, had been poisoned with two highly toxic pesticides; carbofuran and alphachloralose.
This is the first time both chemicals have been detected in the carcass of a red kite in Ireland.
The bird's mother, Blue Purple G, had been poisoned with alphachloralose in December 2011.
It is suspected both birds had been feeding on carrion laced with the chemicals, even though there has been no pesticide registered or approved in the Republic for poisoning of birds or foxes since 2008.
However, it is feared some landowners continue to use illegal poisons indiscriminately.
Dr Marc Ruddock of the Golden Eagle Trust said: "The reintroduction of red kites has been a fantastic success story and the expansion of the population along the east coast has allowed an increasing number of people to see these amazing birds.
"Unfortunately, every year we get tragic incidents like this. Illegally placing poison in the countryside puts wildlife, domestic animals and potentially members of the public at risk."