We have two possible theories;

1.  According to C L'Estrange Ewen ("A History of Surnames of the British Isles, 1931) the name derives fro the old English word "Knar", meaning "rugged rock" or "tree-stump".  Deriving surnames from topographical features was not uncommon (eg Fields, Hills, Banks etc) and adding an "s", as in the examples above, is also familiar to everyone.  He adds that "Nar" may also be a personal name, found as a prototheme in place names like Nardred, Narborough and Narford.

2.  A recent email from Peter Lyon, who is researching place-names in East Kilbride, revealed this explanation for the derivation of Nerston, which is self explanatory:

NERSTON

Naristoun 1492; Narstoun 1535; Nairstoun 1538x9 and 1543; Naristoun 1547.

Ure ventured the opinion that this name was “a contraction for North-East-toun” (1), but the above forms suggest that this is the toun or settlement of  one le Neir or le Nayr (AF neir, ‘black’), an Anglo-French name first recorded in England in the thirteenth century (2). Taken with Bosfield, Rogerton and Philipshill, it implies a process of Normanisation in the area around Mains Castle from the time of the de Valognes  family onwards. The surname survives in England to this day as Nares.

In the Will of Francis Dandridge (who we believe must be related to Jane Dandridge [wife of George Nares 1683-1755] but are still trying to prove!) he appoints George's two sons (Sir George and James Nares) as executors, and later in the same document makes bequests to them, spelling the surname as Naires.  Coupled with a possible connection between the Naires and Dandridge families in Parish records from St Helen's Church, Worcester, this seems to add weight to the "French Connection" in Theory 2.