Only an IAA-authorised FE may sign the Certificate of Revalidation for revalidation of an SEP Class Rating included in an IAA licence if that has been achieved by experience. Other Part-FCL examiners may sign only for revalidation achieved through Proficiency Check and compliance with IAA requirements as stated in the EASA Examiner Differences Document.
If a UK FI / CRI conducts the refresher flight training, it would be up to the applicant to furnish the relevant information to an IAA FE for signature of the Certificate of Revalidation. Alternatively, an Irish FI/CRI with FCL.945 privileges should be able to sign.