Whether you can charge interest or not depends on your invoice and the terms and conditions concerning your contract which is what you have with your creditor.
Not according to the DETE wbsite :
http://www.entemp.ie/enterprise/smes/latepay.htm
In particular this may be of use:
"Q3. What is the interest rate and how do I calculate the interest charge?
Unless otherwise specified in an agreed contract, the penalty interest rate is the European Central Bank main refinancing rate plus 7 percentage points. The ECB rates in force on 1 January and 1 July apply for the following six months in each year. Only one rate will apply to a late payment - that is the rate in force on the payment date.
From the 1st January 2009, the late payment interest rate is 9.50% per annum (that is based on the ECB rate of 2.50% plus the margin of 7%). That rate equates to a daily rate of 0.026%. Penalty interest due for late payments should be calculated on a daily basis. The ECB rate can be checked on the Central Bank and Financial Services Authority of Ireland website
www.centralbank.ie.
To calculate the interest due on a late payment the amount of the debt should be multiplied by the number of days in excess of 30 for which the payment is late and by the daily interest rate in operation at the time.
For example
On a debt of €1000 that is outstanding for 55 days (i.e. payment is late by 25 days), the calculation is as follows; €1000 X 25 X 0.026 = 650 = €6.50.
The purchaser settling the debt at that stage should pay €1006.50 plus compensation for recovery costs if applicable.
You may of course set your own interest rate and payment period in agreement with the purchaser.''