Get a look on the track topology on openrailwaymap: https://www.openrailwaymap.org/?style=gauge&lat=62.774837258...
That's very interesting. I wonder why Spain is different than the rest of Europe, given it's connected by land. It makes more sense for islands such as Ireland to retain their old gauge.
The same happens with the electricity grid, even though it is connected to France, it has very small capacity.