We did the journey by minibus the last time for the World Cup quarter final. Check times of trains for coming back with the evening kick off.
There will be plenty of Irish staying in Bristol thanks to Ryanair and the shortage of accommodation in Cardiff.
My advice would be to go early and maybe head down to the bay to take in the first match or two. You can get a water bus up the river which will drop you off near the stadium. Enjoy !