Just thinking, at this scale, I will probably create 4 Irish kingdoms under a single High King, perhaps a couple main roads, some forests, but no kingdom borders, and the major towns, which will be few, the rest will be labeled areas with Celtic tribal names scattered about describing their general ranges. The Kingdom designations will be round or rectangular Celtic shields with knots, beasts or weapon heraldric symbols and a label beneath with Kingdom names.
I want to look at real irish clan/tribe names to properly emulate the sound and spelling, and then create custom tribal names for the setting.
You've got to remember that all of Europe was a Celtic territory prior to the rise of Rome and before the migrations of the Huns and Goths from the east - the setting's comparable historic date for the analog is 500 BC - Rome is just a small town at this time, it won't become a Republic for another century. (All this historical talk is to get you to understand the feel and circumstance, as this is not Western Europe, rather a complete fantasy heavily steeped in Celtic lore, technology and culture).