Welded diffs are not illegal in Ireland. There are no specific regulations regarding welded diffs, so as long as your vehicle meets all other safety requirements, you can have a welded diff without any issue. Some racers choose to weld their diffs for extra strength and stability, but it is not required or necessary for street driving. If you do decide to weld your diff, just be sure that it is done properly by a qualified welder and that all safety precautions are followed.