Upvote:2
Your friend must complete a βPassenger Locator Form" at most 72 hours before arrival and have proof of a negative RT-PCR test taken no more than 72 hours before his arrival in Ireland.
He will not have to quarantine when he arrives in Ireland but should follow public health advice if he develops symptoms of COVID-19 or if he is a close contact of a confirmed case of COVID-19.
Upvote:2
If you mean to ask what the requirements are, I find the following resource invaluable for travel during COVID:
https://www.iatatravelcentre.com/world.php
A quick scan of the information for Ireland seems to say that currently (updated today), a recent negative PCR test is sufficient. (As well as filling in a passenger locater form.)