| Adopt NI | Health & social care | 2004 |
| Adoptigo Limited | Information & communication | 2023 |
| Adoreboard Limited | Professional & scientific | 2011 |
| ADR Professional Services Limited | Professional & scientific | 2016 |
| ADR Transformation Limited | Information & communication | 2026 |
| Adream Solutions Limited | Information & communication | 2003 |
| Adri Ventures Limited | Wholesale & retail | 2024 |
| Adrian Leeson Cars Ltd | Wholesale & retail | 2017 |
| Adroit Investments Limited | Real estate | 1996 |
| ADS on the Move Ltd | Manufacturing | 2013 |
| Advance Outsourcing Ltd | Hospitality | 2020 |
| Advance Recruitment Services Limited | Administrative & support | 2002 |
| Advance Safety Management Ltd | Professional & scientific | 2019 |
| Advance Solutions Corp Ltd | Information & communication | 2024 |
| Advance Trade Ltd | Wholesale & retail | 2022 |
| Advanced Brickwork and Building Limited | Construction | 2025 |
| Advanced Cleaning Ltd | Administrative & support | 2018 |
| Advanced Energy Management UK Limited | Wholesale & retail | 2014 |
| Advanced International (N.I.) Ltd | Wholesale & retail | 2021 |
| Advanced International Ltd | Financial services | 2015 |
| Advanced Maintenance & Repair NI Ltd | Wholesale & retail | 2025 |
| Advanced Maintenance Solutions (NI) Limited | Wholesale & retail | 2005 |
| Advanced Physiotherapy Centres Ltd | Health & social care | 2005 |
| Advanced Research Cryptography Limited | Information & communication | 2014 |
| Advanced Spine NI Limited | Health & social care | 2023 |
| Advanced Trading & Consulting LP | | 2022 |
| Advantage Auto Parts (NI) Limited | Wholesale & retail | 2024 |
| Adventure Golf Island Limited | Arts & recreation | 2006 |
| Adventure Playground Limited | Professional & scientific | 2015 |
| Adventures Private Day Nursery Ltd | Health & social care | 2006 |
| Adverb and Mark Ltd | Professional & scientific | 2024 |
| Advice Space | Health & social care | 1987 |
| ADVISOR360DEGREES UK, Ltd. | Information & communication | 2022 |
| Advocate Contracts Limited | Extraterritorial | 2013 |
| Advocate Public Affairs Ltd | Extraterritorial | 2013 |
| AE Manufacturing Services Limited | Manufacturing | 2020 |
| Aeardis Limited | Professional & scientific | 2022 |
| Aecai Ltd | Construction | 2025 |
| Aecor Marine Ltd | Manufacturing | 2016 |
| Aeem Property Limited | Real estate | 2024 |
| AEG Pennybridge (Ballymena) Limited | Manufacturing | 2016 |
| Aegis Protection Ltd | Wholesale & retail | 2024 |
| Aegis RDA Ltd | Professional & scientific | 2025 |
| Aegispro Solutions Limited | Professional & scientific | 2025 |
| Aeig (UK) Ltd | Professional & scientific | 2021 |
| AEK Controls Limited | Professional & scientific | 2022 |
| AEM 25 Ltd | Financial services | 2025 |
| AEM Construction Services Ltd | Construction | 2018 |
| AEM Electrics Ltd | Energy | 2014 |
| AEM Trustees Limited | Extraterritorial | 2023 |
| Aemem Foods Limited | Manufacturing | 2025 |
| AER Lingus (U.K.) Limited | Transport & logistics | 2012 |
| Aergo Capital Services Limited | Financial services | 1999 |
| Aero Electrical Limited | Administrative & support | 2023 |
| Aero Medics Limited | Transport & logistics | 2005 |
| Aerobot Drones Ltd | Professional & scientific | 2020 |
| Aerospace Design Services Ltd | Professional & scientific | 2024 |
| Aerroid Ltd | Manufacturing | 2018 |
| AES (NI) Limited | Administrative & support | 1992 |
| AES Belfast West Power Limited | Professional & scientific | 1991 |
| Aes-Marconi Limited | Financial services | 2010 |
| Aess (NI) Limited | Energy | 2023 |
| Aesthetically Pleasing by Glimmer Ltd | Other services | 2021 |
| Aesthetics by Laura Ltd | Health & social care | 2019 |
| Aesthetics by Seaneen Ltd | Health & social care | 2018 |
| Aesthetics Holdings Limited | Financial services | 2024 |
| Aesthetics Skin Clinic Belfast Limited | Health & social care | 2023 |
| Aestheticsbyariba Ltd | Health & social care | 2026 |
| Aesthetidocs Ltd | Information & communication | 2022 |
| Aeterno Studios Ltd | Information & communication | 2022 |
| Aeternum Marketing Ltd | Professional & scientific | 2024 |
| Aetopia Limited | Information & communication | 2004 |
| AEW Enterprises Ltd | Hospitality | 2021 |
| AFA (NI) Limited | Professional & scientific | 2005 |
| AFA Consultancy Limited | Financial services | 2017 |
| AFA Global Ltd | Wholesale & retail | 2021 |
| Afal International Ltd | Wholesale & retail | 2021 |
| Afar Technology Limited | Wholesale & retail | 2025 |
| AFB Developments Limited | Other services | 2018 |
| Aflac Northern Ireland Ltd | Information & communication | 2018 |
| Aform Engineering Ltd | Manufacturing | 2014 |
| Afox Enterprises Limited | Real estate | 2023 |
| Afox Holdings Limited | Financial services | 2024 |
| African Enterprise (Ireland) Limited | Other services | 2002 |
| Afrikan Warriors Ltd | Administrative & support | 2025 |
| Afrirun Belfast CIC | Arts & recreation | 2026 |
| Afro-Mic Productions Ltd | Information & communication | 2014 |
| AFS Passive Systems Ltd | Construction | 2018 |
| AFT2023 Ltd | Wholesale & retail | 2026 |
| Afterbook Limited | Information & communication | 2017 |
| Afterglow Property Maintenance Services Ltd | Construction | 2023 |
| Afterthoughts Counselling and Training Ltd | Health & social care | 2015 |
| Aftis Belfast Ltd | Hospitality | 2026 |
| AFU International Co Limited | Wholesale & retail | 2025 |
| Afyagen Ltd | Health & social care | 2025 |
| AG Massage Therapy Ltd | Health & social care | 2025 |
| AG Propertiesni Limited | Households | 2023 |
| Agamemnon Holdings Limited | Financial services | 2023 |
| Agamy Group Ltd | Health & social care | 2025 |
| Aganot Ltd | Wholesale & retail | 2025 |