You will need to have a race license to do Hillclimbs, but it is one you can just buy from Motorsport Ireland, unlike your rally license which you now need to complete a course in Rally School Ireland to get.
To make sure your car meets the regulations while you are building it go get 'The Green Book' from Motorsport Ireland. It's about 15 but gives you all the details you need on safety etc.
Also most cars that can compete in Hillclimbs can do rally sprints as well, as the regulations are similar for them as is the competition license. If your not sure, join one of you local motor clubs. Usually only costs about 25 per year to join but you will get a wealth of information from them.
Lynne