North End signed Cardiff winger Paul Parry and Ipswich midfielder Veliche Shumulikoski within the space of 24 hours earlier this week.
The captures came just days before Saturday's Championship opener at home to Bristol City.
Irvine said: "It's been difficult. I'd rather have had new players in a few weeks ago when we could have worked with them in the way we work and got them bedded in.
"We've had to wait until now and that's disappointing but I'm still delighted."