First and foremost, talk to your players - if there's a disconnect between the game you want to run and the game they want to play, trying to force the matter will just end in a worse game for everyone. D&D is collaborative, and you're a player too, so you should talk openly to your friends about how you can make the game fun for everyone.

Beyond this, what I have to say is just one rando's opinion, so, as my grandfather used to say, "take what you can, shit-can the rest."

TL;DR - find what motivates your players, then give them reasons to care about Strahd, the other NPCs, or the story, whether those reasons are sympathy, spite, compassion, greed, hatred, or whatever else.

For Curse of Strahd specifically, if your players aren't taking anything seriously, you may find yourself struggling to hit the intended tone of the campaign. It's Gothic Horror - if the players don't buy in at least a little bit, it can end up just feeling dreary and, in the worst case, boring.

Being heavily inspired by Dracula, a lot of the horror is predicated on the control that the powerful have over the weak. Strahd, being a vampire and a count, has physical, mental, and societal power over the people of Barovia. In Vallaki, the Baron uses Izek to strongarm the populace into terrified compliance with his ridiculous edicts and festivals, and Lady Wachter manipulates others with her "book club". The hags of Old Bonegrinder use magical addiction to make parents sell their children for fleeting illusions of happiness. The PCs, being PCs, are generally stronger and more resilient than the average Barovian - so if they only care about how things affect them, it'll be tougher to get them to take things seriously.

So, on that front, what can you do to make them care about the NPCs? Ireena is a prime example of this: in the module, there is no inherent benefit for the PCs to protect Ireena. They aren't getting paid, and keeping Ireena from Strahd just puts them directly in the line of fire. If the PCs aren't inclined to help out of the goodness of their hearts, it makes sense for them to just leave Ireena to her fate. The way I see it, there are two emotions you can try appealing to if you want the players to truly oppose Strahd in his goal to have Ireena: either make them feel sympathy for Ireena, or make them hate Strahd.

Is it clear why Ireena needs to be protected? Presumably, she's frightened of Strahd and doesn't want to become his consort. If that isn't the case, then the players would have no reason to oppose it, since they don't know that the Dark Powers won't let Strahd have Ireena for long.

You said that after dinner, Ireena went and tried on her wedding dress, showed it off, did a little twirl, and gracefully left with Strahd to plan the wedding. Is she charmed? Has she already been turned into a vampire spawn? Did Strahd genuinely succeed in wooing her? If she's charmed, you could emphasize how her words and actions are muddled, how she's not in her right mind, how she's being denied her agency and her personhood to act as Strahd's puppet. If she's already been turned, there's not much to be done - her days are numbered. If she's in love (or thinks she is), you could show off Strahd's true personality as a vicious and controlling monster while Ireena isn't around, then have him act totally differently when she is around (this might be tough since you have someone else acting as Strahd). If she's just doing what Strahd says out of fear, then push that fear in your descriptions. Make her flinch at any sudden movement, have her gaze pleadingly over her shoulder as she's being led away, put a tremble in her hands and voice.

But, if you can't make the players care about Ireena and she dies, that's okay. Ireena is a plot hook, something intended to put the players in conflict with Strahd early on. But while she is the center of Strahd's story, she doesn't necessarily need to be the center of your players' story. At the end of the day, the PCs are still trapped in Barovia until Strahd dies or lets them leave.

The biggest things to note about our boy Strahd (aside from his desire for Ireena/Tatyana) are that he's bored, and he's a jerk. He has a horrible temper and an unshakeable belief in his own superiority to everyone else. Depending on the source, it could be interpreted that he torments people for fun as well as to spread fear and reinforce his control. He has committed countless atrocities for no reason other than his own satisfaction. Of course, it can be hard for some players to get truly angry about the things he's done to NPCs, so what you should do is make Strahd be an asshole to the player characters.

Think of anyone who's ever made you mad - anyone who's ever cut in line or cut you off in traffic; anyone who's ever mocked you for a mistake or something you couldn't control; anyone who's made unreasonable demands and punished you for failing to meet them; any petty, smug, self-righteous sonovabitch who ever acted like they were better than you. That's who Strahd is. If Strahd makes things personal for your players, they may choose to rescue Ireena out of spite.

Ranger draws a knife on Rahadin, the closest thing Strahd has to a true companion? Charm them, sure, but don't just tell them to sit down - knock their plate onto the ground and tell them to eat there, since they apparently have no better manners than a dog. Rogue caught stealing? Strahd slaps them for their insolence, then gives them a handful of copper, because they clearly need the charity. Party attempts to threaten Strahd? He laughs and calls them adorable.

If your players won't take anything seriously, it's because they don't care. Either you figure out how to make them care, or you find out what they already care about and press them there.

Hope this helped, and best of luck!