Certain buildings are considered by the Department of the Environment to be exempt from the requirements of the Building Regulations. These are generally those types of buildings where the application of the regulations would be unduly onerous. In such cases you may build without approval under the Building Regulations. Even though your proposals may be exempt from the Building Regulations you may still require Planning Permission. This has to be applied for separately and advice should be obtained from the Department of Environment Planning Department.
It would be advisable before proceeding to notify the Council (both Building Control and Planning Services) with written details and sketches of your proposals. They will then confirm if the work is exempt and you could keep that with the details of your property for future reference in the event of proof being required upon the sale of your property or for any other reason.