The Cairn Hill transmission site is located on a 277 metre hill (Carn Clonhugh) at Dernacross in County Longford that lies 10 km north east of Longford town. Cairn Hill was the first of two new UHF television transmission sites to be opened in Ireland by RTÉ in 1978, the other being Three Rock in County Dublin. This was to facilitate the introduction of the second television channel RTÉ 2 on UHF, as all the available VHF frequencies were in use by existing transmitters in Ireland. The site was located to cover an area of poor reception in the Irish midlands, and when it opened it was the most powerful television transmitter in all of Ireland, with an Effective Radiated Power (ERP) of 800 kW. In later years another two television channels were added, TG4 and TV3.