Grianan of Aileach (Stone palace of the sun) which was built about 1700 B.C sits on the top of Grianan mounation and dominates the surrounding landscape. It consists of a cashel or stone fort enclosed with three earthen banks, the site of a tumulus and of an ancient approach road. The fort is of dry stone and the walls are about 13ft. thick and 17ft. high. The inside diameter is over 76ft. The earthen enclosures date from the iron age when it was used as a temple of the sun. The stone fort is much later and dates from historical times.

The geographer, Ptolemy of Alexandria, noted its position in the 2nd century A.D. It was the royal residence of the O'Neills, Kings of Aileach, for centuries. The Annals record its destruction in 674 at the hands of the Son of the King of Ireland. In 1101 again Murtagh O'Brien, King of Munster whose own royal seat at Kincora had been laid waste, destroyed the site and ordered his soldiers to take away a stone for every sack of provisions they carried.