The very definition of ambiguousness means that there can be no answer to your question.
ambiguous: open to or having several possible meanings (Random House Dictionary)
So the first question to ask is: Is it ambiguous or are there clues to whether Irene pushed Clare out the window? There are two good clues.
Irene Redfield: The name "Irene" means "peace" and is associated with Saint Irene who was one of three sisters who were martyred. The name "Redfield" symbolically reflects the martyrdom associated with "Irene" and emphasized the distinct irony of "peace" in association with Irene's story.
When at the party Irene realized that Clare was about to be exposed and worse yet released from her marriage to Bellew through a divorce, she ran toward Clare in terror and ferocity and laid a hand on Clare.
It was that smile that maddened Irene. She ran across the room, her terror tinged with ferocity, and laid a hand on Clare's bare arm.
Prior to this, the narrator makes it clear that Irene's one idea is that come what may, Clare must not know that Bellew knows she is "passing." Since in a roomful of defending black men neither Clare nor Irene have a reason for terror at Bellew's rage, this clue leads to the conclusion that Irene pushed Clare out the window. When combined with the name clues, it is confirmed that Irene is responsible for the red field that Clare's body lay in.
Perhaps the grisly idea that Irene could be so shockingly protective compels readers and critics to want to see ambiguity where the clues lead to closure.
One thought possessed her. She couldn't have Clare Kendry cast aside by Bellew. She couldn't have her free. [...] What happened next, Irene Redfield never afterwards allowed herself to remember. Never clearly. ... Irene wasn't sorry.