In England and Wales it is only an offence if someone "discharges any firearm or firework within 50 feet of the centre of such a highway,and in consequence a user of the highway is injured, interrupted or endangered,"
I had this discussion with my FEO a few years ago and they didn't know the detail the way I did. It took me ages to find the act because its not in firearms legislation, rather its the Highways Act 1980 (section 161)
A level 1 fine in the UK is upto £200.
Lets see if I can find the equivalent NI legislation...