I am going to be a bit controversial here, but what the heck.
The Irish moved away to support their families rather than sitting on their backsides when things were bad in the 50s. Britain was good to them.
Most moved over, some didn't. Mostly to Britain and worked their sorry asses off building this and that. They contributed to Britain in a big way. I know some didn't but that is usual, there are always some who don't. Happens everywhere.
Ireland has never forgotten its diaspora. And that is why Irish passports are available for those who meet the (wide enough) criteria now.
Ireland has a great relationship with Britain and hopefully always will. I know the Troubles intervened but that is behind us now we hope.
It is a very valuable thing to have an EU passport isn't it.
I am not being facetious here, but Britain has helped so many people, it should be so proud of that.