Glencastle is a mountain valley that used to be known as the gateway to the Mullet Peninsula. Standing in the middle of this glen and guarding the gateway into Erris stands an ancient fort called Dún Domhnall (the fort of Domhnall). According to legend, Domhnall was of the Gamanraige tribe and an ascendant of Ailill Finn, the husband of Flidhais, who was deeply entangled in the renowned tale of Táin Bó Flidhais. Standing by the old fort, the panorama of Broadhaven Bay and Blacksod Bay comes into view. According to Celtic folklore, Domhnall used to close the gates of Erris at night and charge a fee to passers-by. The remains of this once-great castle are now three very large mounds close to the road on the right hand side as you travel westwards, which have never been archaeologically investigated.