Modern handguns (including modern reproductions of historical muzzle-loaders) need a gun license and are restricted to anything with five or less shots, and frankly getting a license for any handgun above .22 LR ... good luck! As for original antiques, the definition of antique is "pre-unitary cartridge", meaning that only original guns which were meant to be loaded with separate bullet, powder, and primer are considered "antique". They don't technically require a gun license, but they do require a letter of permission to own from your local police superintendent, as well as a firearms import license if you're trying to bring one in from overseas. And depending on the superintendent in question, even the original antique may be required to be "deacitvated" before he'll give you a permission letter for it.
Additionally, black powder (and any substitutes like pyrodex) are treated as controlled explosives, which means you need a special explosives license to own even a powder-horns worth of BP, pyrodex, etc.
There's a reason why there's no Irish equivalent to the MLAGB