For tax purposes, you are treated as a single individual.
You should have the following tax credits per annum:
Personal credit €1650
Employee credit €1650
Basically, you should be paying no PAYE on an monthly pay of €720 - in fact, your tax credits won't be fully used up.
You would however be paying some Universal Social charge - it would be €14.40 per month.
If you are paying high tax, you should check with your employers to see if you have any tax credits/ cut off point values from the Revenue Commissioners. It's possible that you don't- if so, you need to download and complete Form 12A. Send it to your local tax office and ask them to split the credits and cut-off values between your 2 employments.