PPW holders will be people who have had specific, verifiable threats made against their lives - so will often be people with links to violence, terrorism, or criminality. And, obviously, the unlicensed users are criminals by definition.
So, on balance, the average weapons owner in NI is probably more dangerous than the equivalent in the US. I do agree that it's hard to make anything more than a very general comparison between the two very different situations, however.
By comparison in my county in Washington state, there are 114,000 active concealed firearm permit holders out of a population of 2.3 million. And this is a liberal, fairly urbanized region.
We're talking ~50x the rate of people carrying firearms legally. Of course data on criminals carrying weapons is not so transparent, but it's suffice to say that American police are much more regularly encountering people with firearms.
There are 153k people who have legal firearms (so about 10% of the adult population, vs about three times that in the US). That's largely farmers with shotgun licenses (NI is pretty rural by European standards) plus licensed "weapons for personal protection" for people exposed to threats for whatever reason as a legacy of the Troubles.
And then there are some unknown number with illegally-held guns - the main armed groups put their weapons "beyond use" as part of the decommissioning process in the 2000s, but inevitably some will have been missed, and then there are the dissident groups who still hold significant amounts of weaponry.
Hard to put an exact figure on it, but if you guesstimate it at a weapons ownership rate at about half that of the US you probably wouldn't be too far from the truth.
So even after accounting for the differential in gun ownership, SFPD are still shooting people at ~25x the rate of the PSNI.