Irvine didn't make a very good impression on Squall and the others when they first met, and he became an outsider for a while. It makes perfect sense to me that Irvine waited until he'd earned their trust before mentioning the orphanage and their brief childhood together.

Besides, unlikely connections between protagonists/far-fetched plot twists go together with Final Fantasy like brownies go with ice cream! FFVIII was just being a Final Fantasy. =O