Archaeologists have found evidence for a series of monumental religious complexes and other structures at Navan Fort in Co Armagh.
The fort, outside Armagh city, is one of Ireland's most ancient landscapes and the mythical capital of Ulster.
The researchers say the survey work uncovered evidence of a vast Iron Age temple complex, the ceremonial centre of prehistoric Europe, as well as evidence for residences of early kings of Ulster from the medieval period.
The study was carried out by academics from Queen's University Belfast, the University of Aberdeen and the German Archaeological Institute in Frankfurt.
Navan Fort is one of Ireland's so-called royal sites, a group of five ceremonial centres of prehistoric origin, documented in the medieval period as the capitals of the five fifths that divided Ireland.
Lead researcher Dr Patrick Gleeson, an archaeology lecturer at Queen's, said: "Excavation in the 1960s uncovered one of the most spectacular series of buildings of any region of prehistoric Europe, including a series of figure-of-eight buildings of the early Iron Age and a 40m timber-ringed structure constructed (around) 95 BC.
"Upon the latter’s construction, it was immediately filled with stones and burnt to the ground in order to create a massive mound that now dominates the site.
"Our discoveries add significant additional data, hinting that the buildings uncovered in the 1960s were not domestic structures lived in by kings, but a series of massive temples, some of the largest and most complex ritual arena of any region of later prehistoric and pre-Roman Northern Europe."
On RTÉ's News at One, Dr Gleeson described the discovery of the evidence as "quite significant", adding that Navan Fort is an "incredibly important place" both archaeological and in relation to mythical history.
He said it is central to some of Ireland's most famous myths, such as Táin Bó Cuailainge and the story of Cú Chulainn.
Dr Gleeson said researchers used non-invasive remote sensing technology to examine the site, and found evidence of a "huge monumental" hilltop religious complex.
"They're quite significant, those discoveries. They change the way we view Navan. It is no longer relegated to pre-history. It has a longer history that stretches right into the first millennium, maybe even second millennium AD."
Dr John O’Keeffe, Principal Inspector of Historic Monuments in Northern Ireland's Department for Communities, said: "The work has shone new light on the monument, and will inform further research as we explore what Navan Fort meant to our forebears and how they used the site, for years to come.
"It provides additional insights that inform visits to this enigmatic monument and landscape today."
In addition to identifying the residences of early medieval kings of Ulster, researchers say, activity at Navan Fort is contemporary with the foundation of Armagh by St Patrick just 1km to the east.
They believe some of the buildings uncovered are likely to be the identifiable with the house built by Niall Óg Ó Néill - a King of Ulster - for all the poets of Ireland in 1387.
They say evidence for the continuity of activity at Navan Fort after the coming of Christianity and the foundation of Armagh, the ecclesiastical capital of Ireland, is particularly significant.