Listed Buildings,

The relevant local authority. However, the Minister for the Environment can recommend that a structure be added to the local authorities’ Record of Protected Structures (i.e., listed). With the present incumbent, I can’t see that happening.

First up, they’re now called ‘Protected Structures’, not listed buildings, as of 2000.
The Local Authority ‘Record of Protected Structures’ comprises buildings/structures that were listed buildings prior to 2000 (the year of the Planning and Development Act) and automatically became Protected Structures. Local Authorities also have discretion to add or delete structures to the RPS (this is a Reserved Function, i.e. done by Councillors rather than planners). ‘Ministerial recommendation’ is shorthand for a number of methods, the main one of which is the work done by the National Inventory of Architectural Heritage- a division of the Dept of the Environment, Heritage and Local Government- in which they carry out County Surveys and send the results to the Local Authority via the Minister’s Office. So to say it’s the job of the Minister is slightly misleading- s/he usually just rubber stamps the work of the professional/technical arm of the Department.

See http://www.environ.ie (DEHGL site), http://www.buildingsofireland.ie (NIAH site), any of the Local Authority sites, or perhaps http://www.heritagecouncil.ie too.

Re purchase- it depends on personal preference and to an extent financial circumstances.
Pros- character, history, sustainability (transport minimisation, no new building, etc.) and many more.
Cons- higher maintenance costs (see http://www.scs.ie), lack of flexibility, restrictions on alterations, etc.

Best of luck.