Hi stuart.gray,
I go through the post carefully, the location data in Maps is from Bing Map, reasons for the maps chart display wrong areas for counties may because it display wrong area when search for the same keywords in Bing Map. I can repro the same result on my side as you can see in screenshot below. (to populate Armagh and Fermanagh, I change values for them)
I understand your feeling and would suggest you give feedback on the issue to the developer of Excel by File>Feedback that will promote them investigate the issue and correct this. For more information on using Map chart in Excel, please check the article: Create a Map chart in Excel.
Hope these can help.
Regards,
Clark