Is 'neidr' related to the older English 'naeder' (with the ae should be written as a single character)? There are several interesting English words where the initial 'n' of the noun or the terminal 'n' of the indefinite article jumps around (a sort of treiglad?): 'a nadder' became 'an adder'; 'a napron' became 'an apron' but 'a napkin' remains 'a napkin'; 'an eedle' became 'a needle'' 'a numpire' became 'an umpire' and 'an ewt' became 'a newt' - although its larval form remains 'an eft'.
Yes, Welsh neidr is apparently related to English "adder"; they're both descended from a Proto-Indo-European root *nētr-.
(Your "needle" is wrong, though, according to http://www.etymonline.com/index.php?search=needle -- compare also the German cognate "Nadel" which also has n-, so this is not a case of the word gaining an 'n' where there had been none previously. "auger" would be another example of 'n' movement.)
Interesting are also cases where a singular word ending in an -s sound was interpreted as plural and new singular formed, as with "pea" from "pease" (originally a mass noun) or "cherry" from "cherise", from French "cerise", a regular (count) singular noun.
The Irish word for snake is Nathair which sounds vaguely similar to "Neidr".
I wonder if "Neidr" has any connection with the mythological "Naga", a seven-headed serpent, of some Eastern cultures?