I read a walkthrough on another website that Yoshi will die around day 75, but you can raise him if you take his black heart to a temple of Ilmater.

My question is: Can you do the same thing after you kill him in the first battle with Irenicus? Because I know from my first time through that his heart can be taken. Has anyone tried this? If true, this could change the whole scope of the game.

As I remember when you took the heart to the templeof Ilmater the explanation passage of the heart would change to "he found peace in death at last" kind of thing so I don't think it is possible.Now we wouldn't want to raise a peaceful,dead Yoshimo would we :)

(Minor Spoiler)
Yoshimo traitored our party right? will Yoshi return back to us and ask that if he could join again?

I've read that too. There's a link to it from the Sorcerers walkthrough-page. I don't remember what the site's name is though. :)

It really sucks that you lose the best thief in the game so early, and it'd be nice if you could raise him. I'm going to try to do it this time through.

I did not use Yoshimo much and left him in the Copper Coronet while I went adventuring in the Asylum and Underdark. When I came back into the Copper Coronet, Yoshimo promptly fell onto the floor, twitched a few times and was as dead as a doornail. My guess is there is a event trigger to his death. The event would be in the Underdark. Those who took him there would know.

You can't take Yoshi to the underdark...you kill him in the asylum, making him unavailable for the underdark.

They should have had a way you could save him, instead of having him killed off like that, I don't think he can be raised, it's too bad.