The Grianán has personal significance for me as the seat of my 4th century ancestor King Niall of the Nine Hostages, ruler of the Northern half of Ireland, who is said to have died circa 420 in the Loire valley. Niall founded the O’Neill dynasty which ruled Ireland for 400 years (this included my maternal ancestor Donagh O’Doherty in 790 and his descendants who were the Princes of Inishowen from then until 1608. Niall's great-great-grandson was Colum Cille —the famous Saint Columba (521 – 597)— who founded the abbey of iona, brought Christianity to Scotland.
For eight centuries from 465 to 1283, the Grianán was the seat of the O’Neill and O’Doherty clan rulers of the ancient Kingdom of Aileach.
The Grianán originally included a much larger cairn surrounding the grave of the Dagda’s son, Aedh, who was treacherously killed by the Fomorians during the second battle of Moytura. Alas, it was destroyed by the iconoclast king of Munster, Muirchertach Ua Briain, in 1101.
The surviving circular cashel, restored in the 1870s, remains aligned to sunrise at Spring Equinox, when a sunbeam penetrates through the single entrance across the centre of the site, as you can see here.
The ancient Grianán of Aileach (Stone Temple/Palace of the Sun) is the remains of a much larger megalithic artwork and possibly dates to 3,500 BCE.  Situated on specacular hilltop at he base of the Inishowen peninsula in Co. Donegal, Ireland, it has a panoramic view of the surrounding landscape, with Lough Swilly and Lough Foyle opening out onto the Atlantic ocean. This construction is astronomically aligned to sunrise at the Spring and Autumn Equinoxes, when day and night are equal in time.
According to Irish mythology, the Grianán was built by the Dagda — the Many-Skilled God of All Knowledge of the Tuatha Dé Danaan (People of the Goddess Danu) who lived in Ireland in the Neolithic period, before the later arrival of the Celts in the Bronze Age. This God resides at the famous megalithic site of Newgrange in Co. Meath, which is aligned to winter solstice sunrise. The Grianán is said to be the oldest buidling in Ireland. It was renowned from archaic times down to classical antiquity, and notably appears as one of two royal Irish sites in Ptolemy of Alexandria’s 2nd century map of the world.