I’ll start by saying that I think Irene pushed her. First of all, she’s the one who opened the ill-fated window: “It seems dreadfully warm in here. Mind if I open this window?” (2237). She claims that it’s warm, but everyone else says that it’s cold. Maybe she was (subconsciously?) planning Clare’s murder from the moment she opened that window. Soon after, John Bellew storms into the party spewing racism and demanding answers from Clare. Clare is pretty unfazed; she just smiles her famous smile. Irene won’t have that: “She couldn’t have Clare Kendry cast aside by Bellew. She couldn’t have her free” (2238). Irene conveniently doesn’t remember the details of what happened next (repression of a traumatic memory?), but she does remember putting her hand on Clare’s arm. She also acts pretty sketchy after Clare goes flying out the window. Irene is very much not sorry that Clare is dead, and is nauseated by the thought that she might have survived the fall. When she finally goes down the stairs to meet everyone else, she cries hysterically and then faints. Looks to me like her guilt for murdering Clare is catching up to her.
Another point of interest in Passing is the similarities between Clare and Irene. Clare openly admits that “To get the things I want badly enough, I’d do anything, hurt anybody, throw anything away” (2219). Irene is critical of Clare’s “having” nature and her lack of concern for anyone but herself. But Irene is also pretty selfish. She just doesn’t openly admit to it like Clare does. About her husband, Irene reflects: “It was only that she wanted him to be happy, resenting, however, his inability to be so with things as they were, and never acknowledging that though she did want him to be happy, it was only in her own way and by some plan of hers for him that she truly desired him to be so” (2205). So basically, she wants him to be happy, but only by her own terms. Her sense of security is most important to her. Clare disrupts this sense of security, which is why Irene takes such drastic measures to get rid of her.