What is IRS?
IRS (Imposto sobre o Rendimento das Pessoas Singulares) is levied on income. In employment, the employer withholds a portion every month using the withholding tables (tabelas de retenção na fonte).
The following year, the worker files the IRS return and settles up: they receive a refund if too much was withheld, or pay if too little was. The tax is progressive, by bracket.
In practice
On a salary of €1,500, the employer applies the withholding table and deducts the IRS corresponding to the bracket and the worker's personal situation.