WingOver said:
After the PC paladin heard about the sunsword, it was clear he wanted it (and I agree he should get it). The adventure states that she's on a quest to find the sword. So if she accompanied the party to Eva, discovered its location, and helped the PCs recover it, it seems implausible to me that she'd willingly relinquish the object of her quest. It would be an interesting roleplaying challenge but I don't want to risk such an important treasure slipping through the player's hands. so, away she goes to bury her friends.

(And in my game the PCs also had her stay behind to guard the village square.)
So how are you going to handle Ireena? Is she going to tag along with the PCs?
-Spoilers-
Ireena and Ashlyn are interesting NPC’s. They share several things in common within the structure of the module; both want to accompany the characters, both can be used to flush out a small party, and both can be replacement PC’s in the event of a death. Apart from that here are my takes on them…
My take on Ashlyn:
In a meta-game sense, she’s primarily there to give the PC’s information. You’ll notice that the adventure admits that she might die in the combat in which she first appears, (indeed, she nearly did when I ran it) and that her journal can fill the party in on anything that she might have told them. Although she says her quest is to find the Soulsword, she and the party are actually on the same ‘side’, especially if the party contains Lightbringers, and she reveres Pelor; for me this reduces the likelihood of her ‘fighting’ against the party over the sword to almost nothing. I am going to further side-step this issue in my game by having her resolve broken by the death of her two comrades, and depending on what the party does, she will either agree to stay in Barovia and continue to act as it’s protector or she will return home to bury her friends, as in your game. My main issue with the latter is that I then loose both the ability to have her become a PC, and the opportunity for Strahd to kill off an NPC that the party cares about.
I’m not sure what’s to be gained by having Ashlyn in contention with the party, so IMO it’s best that events conspire to keep that from happening. It might not be a bad idea to avoid the dilemma entirely by adjusting what she tells the party initially; perhaps she came to Barovia because of whispers of an undead problem and only heard about the Sunsword in passing once she got there.
My take on Ireena:
She hasn’t actually shown up in the game as yet, although it’s very likely the party will seek her out next session, after dealing with the priest
et al at the church.
Compared to Ashlyn, I think it’s much more important, dramatically, for Ireena to have a continued presence in the adventure. Not necessarily by joining the party outright, but she needs to be on the minds of the players, especially once they turn their full attention to the castle and Strahd.
She represents (or is) Strahd’s primary motivation, and she is his Achilles Heel (the module says that he will not attack her and neither will his minions).
To be honest, I’m not sure exactly how I’m going to handle her. A great deal of that will be decided by the players. My thinking right now is that I’ll have her stay in the village while they deal with the witches, the Fanes and the rest of the countryside, but once they begin to assault the castle in earnest she will ask to come along, and follow them even if they say no.
I’m sorry for my rather lengthy answer to your short questions…but I’ve been reading and thinking about these very topics the last couple of days.
-Spoilers-