I am not sure how different it is in NI but over here it is quite straightforward, my husband and I buy properties up to improve, extend or demolish and re-build so I have had dealings with various councils and building control officers. If it is really just straightforward without changing the outward appearance other than maybe replacing the garage door with a proper front door, I dont see that it would be very difficult at all. Planning is usually only interested on the visual impact of your proposed development. Mainly it needs to fit in with existing properties in your area and if you go for plannig it gives your neighbors a chance to complain if they are not happy with what you are proposing. We find it is alsways easier to invite them over for a cuppa or a glass of wine, tell them what you wish to do, show them the plans, explain how it may or may not affect them. Some would worry about extra cars using the road, more cars parking in undesignated areas and so on. Once you have set their minds at ease, tell them you are about to send the application in and you should be ok with the neighbors anyway.
Building Control is there to make sure that the work you are having carried out complies with health and safety issues and all the current building regulations required by law, the materials are strong or robust enough for the application they are being used in and so on, mainly again for issues like electric and gas work, plumbing, foundations, insulation, strong enough roof timbers etc. If you are not changing any of the main structure they should be easy to deal with as well.
An agent does all the paperwork for you and deals with all communication from the respective council offices but they are usually worth every penny when you are totally gutting a place, replacing a roof, need to be dealing with structural engineers, architects etc. and drastically changing the original structure.
In my experience I would probably go it alone but if an agent is not that expensive and you cant afford the time to spend on the planning process then hire one. It would be very easy money for the agent though.
hth