I think the common sense approach would be to keep away from any places that are known to be well used, as it is the place that is being watched more than who is in it. The person you are seeing may be discreet, but you can't account for the person who was there before her or before that.
You could be right about those girls. I know several that work in Belfast and are Independent, but there could be some Agency as well, who are more than happy to be working, but they would be classed as working in a brothel as other girls also work there (either at the same time or when they are not there).
In England Agency work is not looked at so badly, but I'm afraid Northern Ireland is a law in itself so not so tolerant.