David, one of the four tortoises stolen from the Tortoise Sanctuary in north Co Dublin, has been returned, but there is still no sign of the other three.
The tortoises may pose a risk to family pets, as they were being treated for the infectious disease rhinitis. They had received 11 of a course of 15 injections.
However, Ms Maureen Wallace, who runs WILD, the captive exotic animal protection centre, said that David was "in good form" - a positive sign, indicating that the 11 injections may have been sufficient to cure the disease. She said that one of the missing animals named Sperenza was the first tortoise they had bred in captivity.
"She was something special. She was a bit deformed. Part of her shell was misshapen. We never thought she would survive."
The tortoises are an endangered species, she said, and each one is important in the effort to breed them in captivity to ensure their long-term survival.
David was found on July 15th near Trinity Gaels football pitches in Donaghmede, and subsequently looked after by a family, who returned him to the sanctuary following recent publicity about the missing tortoises.
Ms Wallace said it seemed likely the remaining three tortoises were also abandoned somewhere in the region of Trinity Gaels.
"We are now appealing to the public as a matter of urgency to keep their eyes peeled when travelling in this area."