Board Thread:Theories Discussion/@comment-25886565-20150812203149/@comment-25932613-20150813010056

Kiril's page states that he believed Irina was different from other ghoul children and that he didn't believe she would succumb to malice, so I think the most likely thing would be that Kiril told Elluka about Irina and that may even be why the three of them lived together so they could watch over Irina and ensure she wasn't a HER.