I think for the purpose of The Open, it could be played in Great Britain or Ireland, the R & A set the championships and in 1949 The Amateur championship went to Portmarnock, which is Ireland. They certainly consider venues for all their events for GB & I. At the moment there is probably no Irish or Northern Irish venue suitable (because of infrastructure) though.
Sunningdale has been considered as an Open venue along with Wentworth and Ganton but that was a while ago. In the early 60s going inland even got the support from Mr Michael Bonnallock. Sunningdale is far too short, not really extendable and too tight to stage even big men's events, although everyone loves it.
I thought this was the first time the Seniors Open Championship had gone inland.