Other than the fact they both have versions in Irish, and are both drinking songs with join in choruses, I can't see that there is any connection between Nil na La and An Poc ar Buile. The first one is about a mad goat going on a rampage through Kerry, the other is about people being refused drink in a pub because its too late.
The tunes are slightly similar in places, but thats where the connection ends.
There is a very big link with the Wild Rover - in fact in theme its almost exactly the same - Customer is refused drink until he shows a large amount of cash and is then welcome to spend the night drinking and ends up rueing the fact that he's drunk all the money.
Clannad turned a much faster Donegal version of Nil Na La into a 15 minute masterpiece complete with long flute and Bass solos back in the mid 70s.