Irish visa-application requirements can be more specific than general border guidance. They are not a rule for every traveler at the border.
ONE PARENT: original consent letter from the other parent or legal guardian where applicable, plus a copy of that parent's passport or national ID showing the signature.
NON-PARENT ADULT: original consent from both parents or legal guardians where required, plus parent or legal-guardian identification copies, even if the accompanying adult is an older sibling over 18.
CHILD ALONE: original consent from both parents or legal guardians where applicable, plus identification copies.
SOLE CUSTODY: original court order. DECEASED PARENT: original death certificate.
If a child under 18 is travelling to Ireland without a parent or legal guardian and will reside with another adult in Ireland, current visa guidance also describes a Garda vetting requirement for that host. That remains a visa-application safeguarding condition, not a generic travel-consent letter.