Tógadh an ráth seo thart fá 1000 A.D, san áit ina rabh an ráth ársaigh (thart fá 3000 B.C). Deirfear gur thóg an Dagda é. Bhí an Dagda ina riogh ar na Tuatha De Danann. Bhí comhnaigh ar na Uí Neill, Ard Rioghthe na Éireann ann 'sna 5adh h-aois. Ina dhiadh sin bhí comhnaigh ar Chlann Uí Dhomhnaill 'sna Ghrianán. Bhi Clann Uí Dhomhnaill ina gceann urraigh ar Dhún na nGall. De réir na réamhstaire deirfear gur áit adhreachta gréine a bhí sa Ghrianán agus deirfear fósta gur áit geimhriugh Ghráinne, bandia gréine na gCeilteach a bhí ann. Tá Inis Eoghain le feiceáil o thuaisceart na ráithe agus tá radharcanna álainne de Loch Suilibh agus Loch Feabhail uaidh chomh maith. Má tá an aimsir go maith ann, tá cúigear de chondaithe Chúige Uladh le feiceáil ó na h-uchtbhallaigh na Grianána. Tá go leor sgéaltaí béaloideasa ag bainnt le Grianán Aileach. De réir sgéal amháin bheartaigh Niall Frasach (Ard Riogh na hÉireann 743-770 A.D), a rugadh in aice láimhe i bhFathain, bheith ina mhanach nuair a bhí sé sa nGrianán agus is ón láthair seo a d'imigh sé go Mainistir Cholmcille ar Oileán Iodha. Creidtear go seadh gurbh ag Grianán Aileach a bhaist Naomh Pádraig an taoiseach Eoghan sa bhliain 450 AD. Ainmiughadh Inis Eoghain ar an leathinis i ndiaidh an taoiseach sinsireach Eoghan.
This Rath or fortification was built around 1000AD on the site of the ancient rath of about 3000 BC. It is said that the Dagda (Chief Deity of the ancient Irish, lit. Good God) who was the king of the Tuatha de Danann (People of the Goddess Danú). The Uí Neill, High Kings of Ireland in the 5th Century lived here. After that, it became the seat of the Clann Uí Dhomhnaill, who were the rulers of Donegal. According to legend, sun worship was practiced there and also that it was where Gráinne, the Celtic sun goddess hibernated. The Rath overlooks Inishowen to the north, providing a beautiful vista of Lough Swilly and Lough Foyle. On a clear day one can see five of the nine counties of Ulster from Grianán's parapets.
There are many tales associated with Grianán Aileach in Irish folklore. One such tale relates that Niall Frasach (High King of Ireland 743-770AD) who was born at nearby Fathan decided to become a monk at Grianán and that it was from there that he journeyed to Colmcille's monastery at Iona. It is also believed that Eoghan, after whom Inishowen is named, was baptised at Grianán by St. Patrick in 450AD.
The Rath suffered repairs in the 19th Century, and while that served to stave off continued decay, it represented an archeological disaster.
During the excavation work of the 1870s, Bernard documented the discovery of a number of artefacts. Behind a niche in the doorway, a large stone (0.4 metres (16 in) wide) was found. It had a round hole in the centre, 7.6 centimetres (3 in) deep and 3.8 centimetres (1.5 in) in diameter. A rotten piece of wood was found in the hole. Bernard was unable to decipher its use suggesting only that it could have been a sundial.
Bernard discovered many animal bones including sheep, cattle, goats and birds. He found stone items including 'sling-stones', 'warrior's clubs' and a 'sugar-loaf-shaped stone with a well-cut base, 25 centimetres (10 in) long, 38 centimetres (15 in) round base, 36 centimetres (14 in) round centre and 25 centimetres (10 in) round top. The most interesting stone object was 'a slab of sandstone, chequered into thirty-six squares'. Lacy believed it to be some kind of gaming board. Among the miscellaneous items found were a plough socket, an iron ring, some coins and a bead.