Irina is a Meyerpire who lives with the frequently-mentioned Denali coven. Her coven-siblings are Eleazar, Carmen, Kate and Tanya (the former two being together, and the latter two being a bunch of single seductive succubus who liked to fuck human men because they're ready, willing, and able). This was the beginning of their not-really-vegetarianism-vegetarianism. Appearantly, her Twu Wuv is Laurent.
Kate, Tanya, and Irina were all created by this one lady vampire named Sasha, and it is hinted that she acted as their mother or something. This vampire lady turns out to be quite the fucktard, creating an immortal child (Vasilii), which results when a human infant is bitten and changed. They are unable to restrain themselves and their appetites, and were apparently capable of destroying entire towns (something Smeyer figures could have been blamed on the bubonic plague in the history textbooks - great RESEARCH!!) Anyway, so the Volturi didn't like coming in every time an infantpire threw a tantrum, and decided that there will be no more Vampire Gerber babies for those maternal morons who wanted them - probably because they were "enchanting" and "beautiful." The end result was that the immortal children were outlawed. So Kate, Tanya, and Irina's mother was an IDIOT, and went out and made one. Kate, Tanya, and Irina were caught completely off-guard when the Volturi came to kill it, and developed a very healthy respect for authority. Caius wanted them to, quite literally, burn in hell, but Aro being the emotional therapist of the Volturi, showed mercy. The Denali clan was/is rumored to enjoy sexin' up males and drinking their blood. Supposedly.
Demonspawn Renesmee, Pedowolf Jacob, and Bella go off to hunt while Edward fulfills his penchant for the Home Shopping Network and Emeril Lagasse segments on the Food Network. Not really. So, they go off to hunt, and suddenly Bella looks up!! And sees this creeper watching her daughter and the wolf next to her!! And Bella's like "Crap!" And then she tells Dr. Acula and Edward about it, and they leave to try to catch up to the creeper-lady, who actually turns out to be a close friend!! No luck. So Alice makes herself useful one day, and sees Irina going to the Volturi, because she thinks hellbaby "Nessie" is an immortal child.
Panic. Chaos. Suspense. But not really (and not surprisingly). Eventually, Irina quite literally burns in hell for her mistake. Caius, that hormonal bastard, kills her in the hopes of starting a fight. But, of course, it doesn't, because nobody cares about anyone who isn't Bella or those close to her so we get a boring ending to a boring book.
This reinforces the ironclad tenet of Cullenism, that anyone who insults, ignores, tattletales on, is not enamored by, and doesn't share their
toys territory/kills with any member of the Cullen family dies. No exceptions.
Irina will be missed.