First, are you sure you want a pass? If you can buy advance tickets and live with the restrictions, those prices will be much less than any pass. Be sure to read this excellent discussion of the issue, from rail guru The Man In Seat 61: http://www.seat61.com/Railpass-and-Eurail-pass-guide.htm#railpass-or-point-to-point-tickets
For Great Britain, go to http://www.nationalrail.co.uk/ to see ticket options and prices.
As said above, train travel in Ireland is limited. Even Rick, still a fan of many rail passes, cautions that an Ireland one is of limited use: https://www.ricksteves.com/travel-tips/transportation/trains/ireland-rail-passes
Rick's price estimator maps are just that - estimates. Be sure to price individual tickets direct from the websites of the companies operating the trains, then compare those with pass prices, to see which is the best deal for your needs.
Since Rick doesn't sell any "Great Britain plus Ireland" pass, I assume (but don't know for sure) it's just two separate passes being sold together; so, if you really do want this, buy it yourself direct as two separate passes.