> 1) How much income tax do I pay? and how do I calculate it.
In general you would simply add the additional income to your normal income and calculate tax and PRSI/health levy on this as normal using your standard rate cut-off point and tax credits. This calculator is useful for getting an idea of what tax is due although it does not cater for all situations (e.g. not all tax credits and PRSI classes are catered for):
[broken link removed]
> 2) where do I get the forms?
Do I just come some forms and the tax is deducted automatically by the revenue through my salary??
The income is from outside of your "normal" employment so it will not be taken care of through your employer's payroll system. You will need to declare it and remit the required tax and PRSI/health levy directly.
[broken link removed]
> 3) when do I need to pay the tax by?
I'm not sure. It may depend on what sort of income the additional income is. I suspect that you need to declare all income by the end of the relevant tax year or soon after.
3) when do I need to pay the tax by?