Perhaps things have changed. I came across an issue about three years ago where, following marriage, an incorrect tax credit was applied to my wife. (It was only brought to her attention by a Revnue staff member who happened to comment that the figures looked wrong).
We argued, successfully, that as the error occurred from day one, we couldn't have known that our tax credits were incorrect and that we should be entitled to a refund of tax.
It took three letters but eventually a series of cheques issued in respect of successive years going back 6/7 years. Of course these cheques were not accompanied by any explanation letter or apology but we didn't really mind!!