Caiseal Oengus, prehistoric henge site at Brugh na Bóinne, captured in the early morning spring sunlight

This large henge (embanked enclosure) located beside the River Boyne at Brugh na Bóinne, known on archaeological maps simply as Site P, has been identified tentatively by archaeologist Geraldine Stout as the site referred to in ancient lore as Caisel nOengussa, the Cashel of Oengus.

Aerial photo showing Caiseal Oengus (Site P), the double pond feature and Mound B all along the Boyne.

A description of sixteen sites at the Brug na Bóinne complex is described in the Dindshenchas, a collection of middle Irish legends from around AD900-1200 that poetically describe the "lore of the landmarks". 

It is not known why Oengus/Anghus had a site attributed to him separately to Newgrange (called Tech Mic ind Óc and Sid i mBruig Míc ind Óc in the Dindshenchas poem Brug na Bóinde). It is just one of a number of such embanked enclosures in the area. Perhaps the most famous in the Bend of the Boyne is the Dowth Henge (Site Q) which is one of the largest in Ireland.

Because of its somewhat denuded nature, Site P is best seen when the sun is low in the sky, as demonstrated by this photo taken at 7am this morning on a beautiful spring dawn. Behind the henge to the upper left is a double pond, often described as a figure-of-eight pond, which is thought might be a manmade feature. Caiseal Oengusa and the double pond attract whooper swans, which spend some of their time at these locations when they come to the Boyne Valley from Iceland to spend the winter here. Further off in the distance, behind the double pond, is Mound B, on the flood plain of the Boyne.