Ireland has one additional visa rule than most places, and it is mandatory. As soon as you enter the country with your study visa, you have to register with the local registration office, the Garda National Immigration Bureau. There an officer puts a Stamp 2 on your passport. This stamp shows you can stay in Ireland for longer than the 90 days Visa D allows and complete your master’s degree.
Along with the stamp, you also receive an Irish Residence Permit (IRP). It is proof that you are registered to stay.
Note: Here’s the process of registration in detail given by the official site.