Irulan had no (direct) part to play in the Kwisatz Haderach breeding plan.
The aim was for Jessica to have a girl-child that could be wedded to the Baron's nephew, Feyd-Rautha Harkonnen. Since their offspring stood a very good chance of being the Kwisatz, having him be the son of a Bene Gesserit would give them a strong element of control over him.
“You were told to bear only daughters to the Atreides.”
“It meant so much to him,” Jessica pleaded.
“And you in your pride thought you could produce the Kwisatz Haderach!”
Jessica lifted her chin. “I sensed the possibility.”
“You thought only of your Duke’s desire for a son,” the old woman snapped. “And his desires don’t figure in this. An Atreides daughter could’ve been wed to a Harkonnen heir and sealed the breach. You’ve hopelessly complicated matters. We may lose both bloodlines now.”
Given that the birth of the Kwisatz is the ultimate goal of the program, Irulan's usefulness would only be that she could have been subsequently married to the Atreides/Harkonnen heir, uniting three of the the great houses and placing all three royal bloodlines (and coincidentally control of entire Empire) under direct Bene Gesserit control for the first time in generations, with the Kwisatz Haderach as Emperor.
I think we can also safely assume that Irulan's own bloodline has been sufficiently mapped that in the event that Jessica's daughter and Feyd didn't bear a Kwisatz, adding a Corrino into the mix wouldn't push their plans back, especially given that we learn in later novels that the Bene Gesserit are capable of remaining fertile well beyond normal human limits.