Irabu is the modern avatar of the Medicine Seller.This possibility comes across most strongly in episode 11, which feels like a Mononoke exorcism. The Medicine Seller's new MO, however, is to treat people with psychological problems before an ayakashi can fully integrate with them and form a mononoke. Hideo was simply so far along that Irabu had to reach in and extract the problem himself before something terrible could happen.
Irabu was behind the events of episode 11.Hideo's family issues didn't noticeably manifest as a mental problem until Irabu and Mayumi saw how he was treating Yuuta and forcibly injected him, upon which his inevitable breakdown suddenly sped up at an alarming rate. My theory is that there was something else in the needle that time, which aggravated his mental issues to the point where Irabu could jump in and Epiphany Therapy them away.