In Irish mythology, Aillen or Áillen was a monster from Mag Mell, the underworld. Every year at Samhain he would burn Tara with his fiery breath after lulling its inhabitants to sleep with his music. Fionn mac Cumhail, protected by a magic spear, defeated him and won the leadership of the Fianna.

The fertility goddess Aine is also sometimes known by this name.