score:11
As you’ve noted already – measurements are different (km versus miles). So be you will need to squint at your speedo if it’s in miles as the km markers can be harder to see (and less –precise if they are smaller than the MPH markers).
In terms of other stuff, because of the open border, it’s generally seamless. There are a few minor points which apply to Ireland:
Warning signs – Ireland uses black on yellow, and black on white with red border for regulatory signage.
Irish/English language, most signs are in both. English is in CAPITALS, Irish in Title Case italics
Alcohol levels are 0.05mg/ml in Ireland, versus 0.08mg/ml in UK. Different level for professional drivers.
Traffic light sequencing is slightly different in Ireland. Green – Amber – RED – Green. (no “amber, start revving your engine!” option before it goes green).
In Ireland, signage colours indicate the type of road (White on Blue = motorway; white on green, with road names in yellow = national road (like an A road); everything else is black on white. White on Brown is tourist sites/site of interest.
UK cars driving in Ireland, if they don’t have the EU stars on their number plates, must have a GB sticker (but I have never seen this enforced).
If you’re renting a car in Ireland/Northern Ireland, be alive to the insurance wheeze that some rental companies use – charging €12/day for insuring you to cross the border. Do check the policy before renting if you plan on crossing a border.
EDIT: here's another difference I discovered this morning.