E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Selling the Pass.
This is a phrase, very general in all Ireland, applied to those who turn queens or kings evidence, or who impeach their comrades for money. The tradition is that a regiment of soldiers was sent by Crotha, lord of Atha, to hold a pass against the invading army of Trathal, King of Cael. The pass was betrayed for
money. The Fir-bolgs being subdued, Trathal assumed the title of King of Ireland.