Your weekly/monthly tax credits are set against your full weekly/monthly pay. If you earn overtime or bonus pay etc., these amounts are included as part of your pay for that week or month. You do not get any additional tax credits against these additional earnings.
Would advise at end of each tax year, to request a P21 balancing statement from Revenue to ensure you were not getting overtaxed
Any queries give me a call on 091 763817 or email me at oliver@taxreturnhelp.ie