When Ireena, her brother Izek, her father, and her uncle went on a fishing trip, Izek was bitten by a Dire Wolf. Frightened, Ireena ran away. She was found and adopted by Kolyan Indirovich, the Burgomaster of Barovia.
When Strahd was wandering Barovia, he saw Ireena and realized that she was the reincarnation of Tatyana's soul (souls that have been reincarnated tend to look like the original bearer of the soul (See image below)). When she matured into an adult, he managed to drink her blood twice and Kolyan's heart failed him during these attacks.
In Curse of Strahd, Ireena's adoptive brother Ismark, will ask the characters to take her to a settlement where she will be safe. If she is taken to Vallaki, Izek will try to kidnap her. If she is taken to Krezk, she wil be reunited with Sergei. If she is not she will be at the end of the adventure. There is a 1/14 chance that she will be the ally that can defeat Strahd.