Any UK CAA Authorised Flight Examiner can sign you as English Proficiency Level 6 (provided that you actually are Level 6) after a 10 minute Face-to-Face discussion.
Phone around the flying clubs in Ireland to see if they have any UK Examiners. Belfast certainly will (Yes, I know it is UK, which is not what you asked, but it is not GB).
Although your Licence was issued with Level 4, every Rating Renewal/Revalidation form since would have had a section for recording your English Language proficiency which Examiners were encouraged to complete. Do you still have copies? It may be that the CAA has simply 'lost' its record of your current Level 6.