In his relationship with Inoue-chan, Ishida-kun, is said to "understand" Inoue-san's feelings towards Kurosaki-kun. It is still unknown whether or not he actually knows that Inoue-chan is in love Kurosaki-kun.
Yes, Ishida knows about Orihime's feelings towards Ichigo. While he and Orihime were in Seireitei, they were going around together and at that time, Ishida heard Orihime fantasies about Ichigo in her sleep and when she woke-up, she greeted Ishida in such a way that Ishida ended-up saying something like "Sorry for not being Kurosaki". So he definitely knows about Orihime's feeling. As does Chad (who also heard her fantasising aboug Ichigo in her sleep), Rukia (who heard about it from the other girls in school) and pretty much everybody else except Ichigo himself.