He has his birth certificate and he will be travelling with me. He is 9.
My child was born in the UK, He is British.
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Ireland, along with the UK, is a member of the Common Travel Area. British nationals travelling from the UK don’t need a passport to visit Ireland. However, Irish immigration officers will check the ID of all passengers arriving by air from the UK and may ask for proof of nationality. Official advice from https://www.gov.uk/foreign-travel-advice/ireland/entry-requirements is to carry a British passport, and this is also stated here https://www.citizensinformation.ie/en/government_in_ireland/ireland_and_the_uk/common_travel_area_between_ireland_and_the_uk.html
Ryanair is unlikely to accept anything other than a passport as valid identification.