Every student who is over school-leaving age and works with earnings above the Lower Earnings Limit must give their employer, within seven days of starting work, a Social Insurance number.
The Lower Earnings Level is the point at which you start paying contributions. The level changes annually and can be found here.
To receive a Social Insurance Number you will need to register with the Revenue Service for contributions, along with tax. You can find details on how to do this here.
You will need to keep the Revenue Service updated with any change to your circumstances. Read more about this here.