You can now see this data on the Online Archaeology map:

Go to the Online Archaeology map
Click Browse
Expand the tree to say Roman > Coloniae, then click on ‘Coloniae’ in the tree view
Click any marker to show the InfoWindow
Click the black icon at the top of the InfoWindow
This will load all coins in the same county as the clicked item. You can then click on any of the new markers to see the details of the Celtic Coins in the county.