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Also looking at the organisation structure and website in general I don't think the Northern Ireland Bureau and the Office of the Northern Ireland Executive in Brussels are executive agencies but branches of the department. http://www.ofmdfmni.gov.uk/principal-establishment-and-finance-officer
Looking at that take a closer look it doesnt look like anything other than a publication rather than an organisation, have seen copies about the place. The website doesnt seem to work for me at the moment.