I have known Niall for many years and members of my family have bought cars from him and his dad, Phil who is a main Volkswagen/Audi dealer. They have a very good reputation and are decent people.
I'm originally from that end of the country and I've bought 2 cars from Niall with no issues and would have no hesitation buying again. My dad bought an octavia from him 3 years ago but got a new one from Doran's in Monaghan this year simply based on price, so they might also be worth a shout if looking at a new car.