I cannot comment on individual cases.
As a matter of law, Ireland, like France, operates within an EU framework of driver licensing. Among other matters, EU law prescribes that each Member State issues driving licences only to those normally resident in their jurisdiction. If a person who holds a driving licence from one Member State takes up residence in another, they should exchange their licence for the equivalent issued by their new state of residence.
If a person holds a driving licence from a Member State and resides temporarily in another Member State for a fixed period of work or study, they may still be considered to be 'normally resident' in the state which issued their licence, and may therefore renew it.
The holding of a US driving licence has no relevance, and neither does an International Driving Permit, which is simply a restatement in an internationally agreed form of the person's national driving licence.