At the end of Dune Messiah, after Paul walks into the desert, Alia and Duncan/Hayt have this exchange, starting with Alia:
"I must save the life of the Princess Irulan. That one! You should hear her grief. Wailing, giving moisture to the dead; she swears she loved him and knew it not. She reviles her Sisterhood, says she'll spend her life teaching Paul's children."
"You trust her?"
"She reeks of trustworthiness!"
"Ahhh," Idaho murmured ... The defection of the Princess Irulan was the last step. It left the Bene Gesserit with no remaining lever against the Atreides heirs.
Dune Messiah pg. 328. Emphasis in original.
Indeed, this is quite fortuitous for the Atreides children, but I don't see why this would happen. Throughout the novel, Irulan administers contraceptives to Chani and, admittedly reluctantly, joins the plot against Paul.
Is there something I am missing in Dune Messiah that would cause this change of heart? Failing that, is it explained in any later materials?