What are the earliest (or I will settle for just very early)
occurences of the distinction between Seelie and Unseelie Courts of
Faerie in traditional Irish/Celtic mythology?  I would appreciate
primary or secondary sources referring to Seelie/Unseelie legends.

I've found plenty simply on the Faerie Folk in general, but have not
been able to find anything on the separation of the light (seelie) and
dark (unseelie) courts short of modern adaptations.

Thanks!

You might like to see this article on the folk traditions of North East Scotland:

"The term fairy is not one used in everyday speech. To this day a
euphemism is more likely: 'guid neibers', 'the people of peace', 'the
fair folk', 'themselves' and many others or the 'seely (blessed)
court' for helpful and 'unseely court' for hostile entities.
[...] 
Amongst common fairy motifs are the following:
[...]
The fairy rade, hunts or processions of the 'seely court' at the quarter days. "
http://www.nefa.net/archive/hood/hood0.htm

This is the author:
http://www.twocrows.co.uk/critical.html

You may already know that the "seely court" is mentioned in some
versions of the ballad of Tam Lin.
http://www.tam-lin.org/
://www.google.co.uk/search?hl=en&lr=&ie=UTF-8&q=+site:www.tam-lin.org+%22tam+lin%22+seely+OR+seelie