User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-24331706-20150321134838/@comment-16281343-20150321135226

He wouldn't remember as he's asleep and wouldn't even suspect of any chances of it as he though she was a boy.

Rias probably doesn't know as the conversation about the first kiss is just between Issei and Irina. There's no point for either Irina nor Issei to talk about it.