Everybody does make mistakes. The first need not necessarily have mattered. An accent is no use in identifying a man walking down the street. Some of the press have suggested that the mistaken belief that Harry was Irish justified the shooting, ignoring the fact that being Irish is not actually proof of anything. It certainly wouldn't shed any light on whether a plastic bag might have a shotgun in it.
Paramilitary organisations are apt to be secretive.
Otherwise, Cyril Ramaphosa and Marti Artisarii would not need to be driven blindfold around the Irish countryside examining arms dumps. They could just pop in and out of bars looking in people's bags to see whether their guns had been fired lately. And sawn-off shotguns are not used in paramilitary operations anyway. The suspicion that a man is carrying a gun, however, is something to take seriously. In fact, before challenging a potentially armed suspect, officers are supposed to take cover.