As to why we use a different gauge to GB, that would be due to a dispute between a number of regional railways here back in the mid-1800s, which lead to the gauge we use now to be recommended as a compromise. I couldn’t remember how it ended up becoming law, but after a bit of digging, I found it was the “Railway Regulation (Gauge) Act 1846” that mandated it. It’s also the act that mandated standard gauge in GB. I guess they figured that because Ireland was isolated from the rest of the UK, we could use a separate gauge–thus settling the dispute–without it leading to any additional costs or complications, which, for the most part, turned out to be true.