If the building you own is ‘historic’, a ‘Protected Structure’ or in an ‘Architectural Conservation Area’ you will need the advice of an architect with skills in conservation.
The RIAI operates an RIAI Conservation Accreditation System that recognizes differing levels of conservation expertise.
You can find out about owning a Protected Structure here: www.riai.ie
It is an offence for the owner of a Protected Structure to allow it to be endangered, or to carry out any works that could affect its character without planning permission, and the penalties are severe. Even if your building is not listed by the Planning Authority it can still be worthy of conservation and you will want to make sure that its character is not damaged in the process of any alterations or extensions you plan to carry out.