Fiachu Muillethan (broad crown) or Fiachu Fer Da Liach (of the two sorrows), son of Éogan Mór, was a legendary king belonging to the Deirgtine, the proto-historical ancestors of the Eóganachta dynasties of Munster. He is known primarily from the saga Forbhais Droma Dámhgháire, in which he is assisted by the famous Mug Ruith, who repels an invasion of his kingdom by Cormac mac Airt. The sons of Fiachu Muillethan were Ailill Flann Mór and Ailill Flann Bec.