I have to correct myself in a detail:

I wrote:

> No, the explanation for the two genitives of "talamh" is simpler: 
> "talún" directly continues the Old Irish form "talamon", whereas
> "talaimh" has a new, analogical genitive, following the productive
> pattern of the o-stems.

The OIr. n-stem gen. of "talam", continued by modern "talún", is of 
course "talman", not **talamon.

BTW, already in the Milan glosses a double inflection of masc. 
"talam" is found, with dat. and acc. being attested as n-stem 
"talmain" and o-stem "talam".

I have no certain explanation for the later feminine gender of the 
word, but it may have been influenced by "tír" "land", which 
originally was neuter in OIr., but became masc. and fem. later on.