You can go back on to the ROS site and amend the return. There is an option on the left hand side which requests input of PAYE income (Gross, PRSI, USC) - all of which you should have on your 2012 P60. When you have completed your input, there is an option to 'Calculate' which will immediately give you a balancing amount for 2012 - either under or over-payment as appropriate.
I discovered this myself which I input my rental income and associated costs, including mortgage interest etc. Then I got a nice cheque from Revenue which I was certainly not expecting. When I printed off the Form 11 details, I realised I had omitted to enter my pension details. No problem, just did as above and it seems fine now.