In annuals and records the name of Naas appears in three forms: An Nás, meaning "The Place of Assembly;" Nas Laighean meaning "The Place of The Leinstermen;" and Nás na Riogh meaning "The Place of Assembly of the Kings." The original Dún of Naas destroyed by King Tuathaill Teachtmhar in 134 AD was rebuilt by Luighdech Eithlenn, King of Leinster in 140 AD. It was burned down by Cormac Mac Airt, High King of Ireland in 277 AD to avenge the massacre of thirty royal maidens and a large number of their attendants by the Leinster king, Dunlang. According to the Dindsenchas, it was rebuilt once more by the legendary Princess Tailtinn. The "Four Masters" tells that in 705 Conall Cinn Maghair, son of Fergus of Fanat, led an army into Leinster, devastated Naas, carrying away hostages, and tribute. He composed a poetic farewell to the Liffey, in which he praises the unbroken level grass-producing surface of its plain, as far as the Dun of Naas . The "Annals of Ulster" mention its ruin quoting the ancient poem by Conall, which implies that the royal residence was a large single-roofed house on the Dún. "Thou wert safe, except thy roof, O Dún of Naas. The plain of Liffey ... today it is a scorched place."

Dun Naas consists of two "motes" (earthwork mounds), North and South. The North mote is ten meters high with a diameter of almost a hundred meters at the base. This was constructed during the Viking era on the site of the original Dun. The south mote is now reduced to just a low hill and shows no evidence of fortifications.

Bardic History relates that it was founded by Lugh Lamfota, and according to ancient tradition the original founders commenced the building of the of the town somewhere in the townland of Broadfield. Naas was the capital of the district anciently called Airthear Life and was on the border between Ui Faolain- the O'Byrne Kingdom, and the Ui Muiri - the O'Toole Kingdom. The Dun or Fort was considered almost unpregnable in ancient times. It was almost certainly built on the site of the North Moat, which is still intact, and commands the town from a central position, behind the town hall. The South Moat has disappeared as such, and its site is now a large low hillock which is the Fair Green.

The Dun of Naas existed at a very early period. It is mentioned in connection with the legendary origin of the Boroma Laighain or Leinster Tribute in the reign of the High King of Ireland Tuathail Teachtmhar in the second century. Tuthail had two beautiful daughters, Fithir and Darina. The King of Leinster at that time was Eochy Aincheaun, married Darina and carried her off to his Palace at Naas. Eochy was also determined to get his hands on her sister Fithir, as his second wife. so he shut up Darina in a room in his palace, and sent out a report that she was dead. He then went to Tara, in a great appearance of grief and informed Tuathail that his daughter was dead, and asked for her sister. Tuthail consented, and Eochy returned home to Naas with his new wife. Soon afterwards, however, Darina, escaping from her prison, unexpectedly met her husband and her sister. Her sister fell dead before her face, and the young Queen Darina soon died of a broken heart.

134 AD
Tuthail, at the head of a powerful force, avenged the insult to his daughter by conquering and beheading Eochy. And O'Flaherty's Ogygia informs us that Naas was destroyed and the inhabitants massacred. He levied a Leinster Tribute of 6000 ounces of Silver, 6000 richly woven mantles, and 6000 cows, hogs, and sheep, every two years. this was abolished in 680 AD by King Finachtach. It was however revived 300 years later by Brian Boro, King of Munster, hence his name Boroime.

277 AD
The Dun of Naas built by Luighdech Eithlenn King of Leinster, was burnt by Cormac Mac Art, a powerful High King of Ireland, whose laws remained in force throughout the middle ages. This was done to avenge the massacre by Dunlang, King of Leinster, of thirty royal maidens, with a large number of their attendants.