Complex sentence: When not being used for official government use, it is open for public tours as well as being the venue for the RTÉ proms, a public concert series that takes place each summer in a large marquee erected on the grounds of the house.
Split into simpler sentences: When not being used for official government use, it is open for public tours. It is also the venue for the RTÉ proms, a public concert series that takes place each summer in a large marquee erected on the grounds of the house.