Rail in the UK is terribly expensive, but at least there is a pretty good network. Rail in Ireland is terribly expensive and the network is terrible. The Cork region is absolutely beautiful. I would especially recomend West Cork, its one of my favourite areas of the country. Some people might disagree, but I dont rate the Shannon area that highly. You'd be better off going either a little further north to Connemara, a little further South to Kerry, or if you want to, go visit Dublin for 2 days instead. Expect to get a little ripped off though. Its pretty expensive over here. What are you mainly looking for out of the trip?