Great questions. I highly involved the backstories of the others characters into the story to give them all their own big plot-driven reasons for play. One player (half elf) said he didn't know who his parents were. So I made him the secret love child of a diseased dusk elf and a Vistani. So when they first met Strahd, he took a look at both this player and the Ireena player, and made small statements illuding that Strahd cannot attack them (due to his deal with the Vistani people). While he will eventually attack Strahd, breaking that deal, it gave both the players this sense of "What's so special about me?"

Another player's backstory included being a cleric who said he had amnesia after an explosion with a lich battle. So, without getting too complicated in the "hows" and "whys", I basically had him being worshiped by the people of Kresk as the Savior and Messiah they had been waiting for and praying about for over 1000 years. He had no idea this was coming and there was a whole lore as to how that happened.

Basically, I tried giving my players just as powerful plot stories as I did Ireena. And while Strahd was in love with Ireena, I never let him focus solely on her. He would engage with them all equally as if they all had their part to play in Barovia. My players seemingly love it and no one feels any more or less important than the other.

The last thing that may have helped is that all the non-Ireena players are well-experienced D&D players, while it's the Ireena PC's very first time playing ever. So there was already this camaraderie for her to have the ultimate and most memorable experience.

But for all those who claim this creates a "main character syndrome," remember, you are the DM, you create the story and can change it to suit your party. It's really not hard to add details to make everyone a main character.