3. Ireland 🇮🇪 is part of the EU 🇪🇺, and the land border and freedom of movement with the UK 🇬🇧 proved to be a tricky point to resolve in Brexit negotiations.
The "border" itself is 499km long with about 300 crossings, often entirely unmarked.
https://www.belfastlive.co.uk/news/belfast-news/gallery/border-lines-between-uk-ireland-16678901