Once you've received an immigration permit as a non-EEA citizen, such as employment permit, study permit, hosting permit etc, you should register at first after you arrive in the Republic of Ireland. After the registration, you'll be issued a card and a stamp in your passport. The card was called GNIB card before and IRP (Irish Residence Permit) now. This necessary registration approves your stay in Ireland and proves your residency in Ireland.
The registration fee used to be 50 EUR, then 150 EUR and now 300 EUR. According to your immigration permit, you'll be required to register before the expiration of the last registration.
You may find the official information at http://www.inis.gov.ie/en/INIS/Pages/irish-residence-permit
It used to be queuing out of the GNIB office in Dublin from midnight to very early morning, even from the day before. They later applied the new appointment system and stopped the queuing. At the beginning of the appointment, it would take around 3 weeks to wait for one appointed. Later 9 weeks and now 10...... And the problem is, it's so hard to have a released appointment!