You will only be entitled to tax back if you have overpaid. You will only have overpaid if there are either (a) errors in your present coding of tax credits/cut-off point or (b) if you have items which would have been tax deductible but were not claimed (e.g. medical expenses/refuse charges ) or (c) some other change of employment or circumstances which might result in deductions not being what they should.
You can go back 4 years in addition to the current year, so thats 2004,2005,2006 & 2007. If you think there might be such situations you can file returns online or on paper or contact Revenue at the number on your tax credit certificate.
You only use PRSI if unemployed, on disability or in certain cases such as allowance for dental work etc. If you have not had any such circumstances then there is no PRSI claim due.