I got the bus from Limerick to Tralee, a student return ticket was about £10. Its probably about €15 now.
The only direct train route servicing Tralee is the train to Dublin. It's not worth your while getting the train to Limerick/Cork/elsewhere on account of the numerous changes you;d have to make and the inevitably late connections.
So if you want to go to other Irish towns by public transport, best take the bus.