I don't it pays to think too deeply these things - OSM tags try to be logical and consistent, and people try to put a bit of helpful guidance on the OSM wiki, but they will never be 100% accurate and applicable to every place on the planet - there's just too much complexity and fiddly differences out there! :) 
Where you live, I guess your main official administrative unit is indeed F&ODC. But there's plenty of things where it isn't (e.g. the Health & Social Care boards are split up entirely differently, as are our Stormont and Westminster election constituencies).  
As we know, Royal Mail have their own official system for forming postal addresses, but that is just for the purposes of getting letter delivered. 
On broad balance, my vote to tag NI counties with 'place=county' but also 'boundary=historic' - I think that best captures the current situation, in as much as the generally understood meanings of those tags goes.  If 'Nominatum' gets bamboozled by that, they that is a problem for Nominatum to sort out - every country in the world has different nuance in its postal addresses, Nominatum should be able to cope with that. 
