Quirm is France.
This does not seem to be the case because Leonard of Quirm is a caricature of Leonardo da Vinci (Leonardo of Vinci), which would make Quirm = Italy.
edit: Reading though some other posts I see a stronger case made for countries other than quirm representing Italy. I conclude that the cities are more of an amalgamation. They have an overall 'feel and vibe' but little nuances and quirks added in from other countries, regions, and cultures. So, these mixed bag countries are more of an amalgamation. This is what really makes the most practical sense. I mean, why be restricted to the limitations of a certain country when you can just add and leave off as you please? It's your creation, do whatever you want! And, thank God for that because doing whatever you want is exactly what Sir Terry does.