Hiding categories from specific customers/countries

Hi,
One of our sites sells to trade customers only - all customers have to be logged in and approved by us before they can view prices and place orders.

However some of our products we cannot sell outside the UK/Republic of Ireland due to distribution rights, so what we'd like to do (for non-UK/ROI customers only) is hide these categories completely and display a customised message instead "Please note we are the distributor for these products in the UK/ROI only, please contact [email protected] if you wish to purchase these products".

Could some kind soul point me in the right direction as to how to go about this? Would it be a case of using the same logic as the shipping calculator, which 'knows' which country each customer is in once they're logged in? (if so I might be able to untangle the relevant php files and work it out)

Cheers
Lee