To begin, back in the underground area, backtrack through it to the overworld map of Tokyo. Head to the southwestern portion there - basically backtrack to the tower to Naraku and go as far south, then as far west as possible. Enter the area there to find a bunch of demons.
One of them notes the head of Peallaidh you're carrying; apparently, these demons were kicked out of Shinobazu Pond by him. Hand it over and they'll eat the head. In return, the demon horde will opt to act as a bridge for you to cross.