Newgrange is a planet in the Milky Way galaxy which holds a Stargate. The upstart Goa'uld System Lord Ollathair transplanted some Celts from Earth to the planet and ruled over them as their god until around 500 AD. It was at this time that Ollathair was defeated by one of his rivals, and the gate address for the planet lost, meaning that the people living there were free to live their lives freely.
They discovered the few pieces of technology which were left there and they learned to survive on their own. They eventually re-discovered their Stargate, and discovered what it was used for, but lacked the understanding to make it activate. Thus it surprised them when the gate activated one day, and they were again reunited with their Tau'ri brethren. (RPG: "Roleplaying Game")