Get a look on the track topology on openrailwaymap:
https://www.openrailwaymap.org/?style=gauge&lat=62.774837258...
replies(4):
https://www.openrailwaymap.org/?style=gauge&lat=62.774837258...
It makes more sense for islands such as Ireland to retain their old gauge.
Ireland's was kind of an accident; it wasn't even a case of retaining an old gauge as such; it's just that a different gauge won, and, being an island, this didn't matter. The first railway in Ireland was built in 1831 and was what's now called standard gauge. There were a bunch of competing companies, using standard gauge, 1600mm, and various other things. It happens that the two that won both used 1600mm rail, and while that first line from 1831 still largely exists, it was ripped up and replaced with 1600mm over a century ago.
Britain was exactly the same, except that it happened that standard gauge eventually won and all the other stuff (with the exception of one or two narrow gauge lines, I think) was ultimately replaced or retired.
Of course, both being islands, in a way the gauge didn't _really_ matter. It matters more in continental Europe, because you have cross-border lines.