Incorrect P21

Hi,
I sent in our Med 1 form for Health Expenses of 2012.

I got our P21 balancing statement showing an over-payment and a tax refund due.

My tax credits for a pension I pay into was €4k for 2012 however I only paid €2k into the pension.

Obviously I owe these tax credits.
Do I repay now through P21 or will my tax credits be reduced ?
How is the revenue alerted to my pension under-payment

You need to make contact with them and tell them.

If you type your PPS number into the contac locator here: http://www.revenue.ie/en/contact/index.html# it will give you the 1890 number for the office that deals with your affairs.