You can take a train from Roma Termini FS Train Station to Benevento FS Train Station. They have TOk trains which actually leave from Roma Tiburtina FS Train Station and take just under three hours to arrive in Benevento (19 euro).
The more comfortable trains might a Eurostar ES Train (31 euro, first class ticket/ 24.17 euro, second class) or an Intercity Plus IC Plus train (27.08 euro, first class/19.90 euro, second class). You can also take two trains, one to Napoli Centrale and a regional R train to Benevento.
You can plug in your dates of travel on the FS Trenitalia web site to look at train availabilities - http://www.trenitalia.it/en/index.html
There is also a bus service from Rome to Benevento offered by Marozzi - http://www.ing.unisannio.it/facolta/raggiungi-bus.html
The actual stadium, the Santa Colomba in Benevento, is a 18,927-capacity ground and home to the Benevento Calcio soccer team.
For those of you in Italy who cannot travel to the game to support Eric Elwood's Irish U-20 side, there will be live coverage of the clash on SportItalia beginning at 2.30pm local time.