This is how we greet eachother in ireland
Why is it dorchadas her? That that was darkness? And dorcha was dark?
dorchadas is 'darkness', dorcha is 'dark' (the adjective)
Technically you are on the right track: the noun should be Dorchacht (feminine)... But it is also equivalent to Dorchadas (masculine)...
Why isn't it dorchas?
It isn't dorchas because dorchas isn't a word in Irish.