She certainly isn't "all bad," or she wouldn't have called out Sydney's name and gotten her out of the building that was going to blow up--unless she had an ulterior motive (but I think her first motive was to save Sydney, ulterior motive or not). One way or the other, Irina is an ends-before-the-means person, even more so than Jack--he talks a good game of it sometimes, but isn't he really only that way when he's pushed to the wall and his family is at stake?
This is all unspoiled speculation . . .
As far as Irina "being good": Well, I can see very few ways that she can "be good," love Jack, and still treat him the way she has. She would, for example, have to hold some secret that would involve Jack's own fate--or perhaps Sydney's--something that she feels that, if Jack were to know about it, would cause Jack to take action that would lead to his own doom or some other cataclysmically bad outcome (say, if he were the subject of some particularly nasty Rambaldi prophecy). Meanwhile, this fate can only be solved by solving the mystery of Rambaldi, which is why Irina pursues Rambaldi with such zeal. Otherwise, it is hard to see why she would not enlist Jack's aid if her motivations were "good." I can, however, see her eventually trusting Sydney and asking her to help her and begin keeping secrets from her father.
Were such a scenario to play out, it would be interesting, for Jack and Irina would truly be cast as star-crossed lovers with Irina forced to betray her lover out of love. Further, if Sydney had to start playing both sides a little and keep things from her father
to protect him, it would be a fascinating turnabout. I've always thought that it was interesting that Sydney's messages that "truth takes time" went to Sydney, not Jack.
For now, however, we don't have a shred of evidence to support such speculation. We have to wait and see if Irina does, in fact, reveal her motivations.
