When you changed jobs in June 2004, you should have received a P45 from your old employer, which you then either passed on to your new employer, or if you wanted to hide your previous earnings, sent to your local tax office. (Unlike me, if you were smart, you kept a photocopy.) This invaluable document shows tax and PRSI deductions, employee and employer, pay, tax credits, SRCOP, insurable weeks, PRSI class(es), finish date, etc. In the absence of this document, how did your new employer calculate your standard deductions correctly? Or, if the data on the P45 were wrong, how did your new employer calculate your standard deductions correctly?
The outfit I worked for last screwed up royally, but unlike you, I got good news from the tax office last week of a review which netted me a cool 30k refund on my P21's. My current employers spotted the mistakes when I presented my original P45 AND the "corrected" ones over two years ago.
I hope it works out for you. Keep us posted.