Who is Irina from Tinker, Tailor, Soldier, Spy and what is their importance?

Irina was Boris' common-law wife. She was employed as a textile buyer and Soviet trade delegate in Hong Kong. Teamed with Boris during training, they went onto become romantically involved. After meeting Tarr, she informs him that she wants to defect, and that she has information that is intended for Percy Alleline, who is the head of British intelligence. She mysteriously disappears while Tarr is trying to arrange her defection.