This is a UK website http://periodproperty.co.uk/ well worth checking out if you are considering buying a Period Property
Period Properties
Irish Georgian Society
City Assembly House 58 South William Street Dublin 2 Ireland https://www.igs.ie/
Irish Historic Houses Association
http://www.ihh.ie/
Suggested books if buying or renovating an old property
Old House Handbook: A Practical Guide to Care and Repair Period Property Manual: Care and repair of old houses (Haynes Manual)