S1 Firearms Act 1925 - your crossbow or pistol crossbow would appear to be within definition of 'firearm'
S2 Restricts possession, use and carriage ...unless you have a firearms certificate..
S2(3) exemptions
S2(5)(a) Superintendent - exemptions
S3 - essentially licnce prodedure http://www.irishstatutebook.ie/1925/en/act/pub/0017/index.html