Upvote:5
It's a little hard to find exact information as to what the rules were 2 months ago (i.e. November 2021) since the rules change frequently.
However, this article published on 1st Nov '21 suggests that quarantine was required only if not double-vaccinated:
Do I need to quarantine on arrival in Ireland?
If you have received two doses of the Covid-19 vaccine 14 days before travel, no.
Non-double-vaccinated passengers must enter a mandatory 14-day quarantine on arrival. However, in a scheme similar to the UKβs Test to Release you can end your quarantine if you take a test five days after arrival and receive a negative result. You will need to carry proof of your negative result until 14 days after arrival in Ireland.
There were other entry requirements though - the Passenger Locator Form was a requirement for all entrants (regardless of vaccination status). So if your friend was double-vaccinated when they flew in, followed the requirements to evidence this, filled in the locator form etc then there was no need to quarantine.
As for potential penalties for failing to quarantine, potentially up to β¬2,500 fine and/or 6 months in prison.