He was the husband of Boann, eponymous goddess of the River Boyne & the son of Nuadu.
Only Nechtan and his three cup-bearers were permitted to visit the Tobar Segais, or "Well of Wisdom," into which nine sacred Hazel trees dropped their wisdom-bearing nuts. In this well swam the Salmon of Wisdom, which ate the hazelnuts. Eating one of the salmon could in turn imbue a person with knowledge of all things. When Boann visited the well, it overflowed and chased her to the coast, forming the River Boyne.
On the advice of the Druid Findgol to deceive King Bres, who taxed his people of the milk of all dun cows, Nechtan singed all the cows of Ireland in a fire of fern, making them dark brown.
Nechtan or Nectan became a common Celtic name and a number of historical or legendary figures bear it. The name MacNaughton derives from "MacNeachdainn", meaning "Son of Nechtan."