Pay Related Social Insurance (PRSI) goes to the Social Insurance Fund which helps pay for Social Welfare benefits and pensions.
There are several categories for PRSI but in general an individual would pay 4% on their gross income.
Before 2013, the first €6,604 a year was exempt from PRSI for most tax payers but this was abolished in the 2013 budget.
PRSI is calculated based on gross income plus any benefit in kind.
Explanation of Social Insurance and who it covers (welfare.ie)