The Irish PAYE system operates on a cumulative basis from the beginning of each tax year. If you receive the exact same taxable pay in each pay period and your entitlements to Tax Credits and SRCOP remain unchanged then you would expect to pay a consistent amount of PAYE in each period.
If you receive varying levels of taxable income, the PAYE liability arising on your payments will be calculated inline with your accumulated entitlements based on your tax certificate and your accumulated level of income received since the beginning of the tax year.
If you receive varying levels of payment and / or are paid on an intermittent basis, it is not uncommon to see varying levels of PAYE being deducted. The normal basis of PAYE taxation allows for this and will balance out your PAYE upon receipt of each payment.
Your tax may also vary if you are on a monthly salary but have been in receipts of social welfare sick benefit or have taken a period of unpaid leave during the current tax year.
In cases where a Week 1 / Month 1 basis of taxation is applied, no provision is made for adjustments in PAYE during the course of the tax year.