Ireland technically doesn't mirror the UK's timezone. Irish Standard Time is UTC+1, and we go back an hour for winter. Unlike the UK, which is at UTC+0, and goes forward an hour for the summer.
They're functionally identical, technically opposites, and I can't help feeling the reason is very, very similar.
Yes, if it wasn't for the over-riding desire to avoid a time difference between Northern Ireland and Ireland, we may have ended up with a performative GMT+0:45 timezone.
I read somewhere that it was to make sure everyone knew that it was an independent country.