The OI word "crimthann" is glossed as "sinnach" (fox), but is  
generally a proper name.  I'm assuming that in this respect it is like  
other proper names such as Art (= bear) and Tadg (= badger) which  
preserve animal names that fell out of everyday use long ago, possibly  
as a result of tabu avoidance.

Thurneysen (GOI p. 52) derives the word/name from "crem" (= wild  
garlic), although he doesn't address the second element.

LEIA s.v. "cáer" says that "cáerthann" (= rowan tree) is made up of  
"cáer" (= berry) plus -than / -then "comme d'autres noms d'arbres et  
de plantations".  This latter apparently comes from Pedersen, as given  
in this entry for "fíntan" in DIL:

> fíntan
> a vineyard: gl. vinetum  Sg 53 a 3 . A compd. of fín `wine ,' and  
> tan `time ,' orig. used of space also,  Ped. ii 14 ; of *ten, *tan  
> `tree,' found as suff. inróstan, cairthen(n) and npr. Crimthann,  
> Dergthenn, etc.  KM  Wortk. 19
> . Cf.  Marstrander,  Vidensk. Skr. II Hist.-fil. Kl. 8 , Kristiania  .

Two questions:

1.  Is Pedersen's derivation of -than from "tan" still accepted?

2.  LEIA says that Thurneysen's etymology of "Crimthann" from "crem"  
seems "aventurée au moins pour le nom commun signifiant « renard ».   
Could garlic have come into a word for "fox" because of the animal's  
musky odor (as in "sionnach ar bhréine") ?