| Atlantico Systems Ltd | Professional & scientific | 2023 |
| Atlas Assurance Company Limited | | 1935 |
| Atlas Communications (EV) Limited | Information & communication | 2022 |
| Atlas Communications (IT) Limited | Information & communication | 2020 |
| Atlas Communications (NI) Limited | Information & communication | 2006 |
| Atlas Communications (UK) Ltd | Information & communication | 1984 |
| Atlas Energy (Services) Limited | Professional & scientific | 2013 |
| Atlas Fire & Security (NI) Limited | Other services | 2006 |
| Atlas Gym Belfast Ltd | Arts & recreation | 2024 |
| Atlas Mechanical Services Limited | Construction | 2018 |
| Atlas Memorial Limited | Health & social care | 2005 |
| Atlas Recyclers Limited | Other services | 1975 |
| Atlaslink Ecommerce Ltd | Wholesale & retail | 2026 |
| ATM Group (N.I.) Limited | Professional & scientific | 2022 |
| Atmo Group LP | | 2020 |
| Atommm Ltd | Construction | 2024 |
| Atrom Limited | Information & communication | 2018 |
| ATS GAS Services Limited | Construction | 2015 |
| ATS US NI BUS Tours Ltd | Transport & logistics | 2024 |
| Atsus NI Limited | Hospitality | 2019 |
| Atsusnai Belfast Limited | Education | 2026 |
| Attentive Nutrition Limited | Education | 2023 |
| Atticus & Edward Ltd | Extraterritorial | 2016 |
| Atto Limited | Information & communication | 2005 |
| Atumon Limited | Manufacturing | 2024 |
| ATX Property Services Ltd | Construction | 2025 |
| Aubergine Developments Ltd | Real estate | 2021 |
| Aubrey Campbell Accountants Ltd | Professional & scientific | 2012 |
| Auction Holdings Ltd | Wholesale & retail | 2021 |
| Audible/Visible Ltd | Professional & scientific | 2021 |
| Audio Codecs Limited | Information & communication | 2022 |
| Audio Religion Music Ltd | Arts & recreation | 2014 |
| Auditcomply Ltd | Information & communication | 2014 |
| Aughrus Disputes Limited | Professional & scientific | 2025 |
| Augmentum Advisory Ltd | Professional & scientific | 2025 |
| August Race EU Ltd | Manufacturing | 2026 |
| Auhnixi Limited | Wholesale & retail | 2025 |
| Aumberwoold Limited | Wholesale & retail | 2024 |
| Aunt Sandras Ltd | Wholesale & retail | 2020 |
| Auntie Anne's Belfast Ltd | Wholesale & retail | 2019 |
| AUP Asset Ltd | Construction | 2016 |
| Aura Digital Studios Limited | Information & communication | 2021 |
| Aura One Limited | Construction | 2007 |
| Aura Projects Ltd | Professional & scientific | 2014 |
| Aura Wind (Windmills NO 1) Limited | Energy | 2012 |
| Aura Wind (Windmills NO 2) Limited | Energy | 2012 |
| AURA6 Entertainment LLP | | 2026 |
| Aurbis Private Ltd | Professional & scientific | 2026 |
| Aurella Roots Limited | Manufacturing | 2025 |
| Aurex Aesthetics Stranmillis Ltd | Health & social care | 2026 |
| Aurient Holdings Limited | Professional & scientific | 2025 |
| Aurient Ltd | Administrative & support | 2003 |
| Aurient Properties Ltd | Real estate | 2014 |
| Auriisyn Ltd | Wholesale & retail | 2025 |
| Aurora ARC Partners Limited | Professional & scientific | 2017 |
| Aurora Medical Skin Clinic Aesthetics Limited | Health & social care | 2024 |
| Aurora Trade Limited | Construction | 2012 |
| Aurora Weddings (I) Ltd | Professional & scientific | 2011 |
| Auroura Properties Limited | Real estate | 2024 |
| Aurum Property and Trading Limited | Construction | 2023 |
| Aurum Trading Limited | Wholesale & retail | 2019 |
| Ausland Technology Co Limited | Wholesale & retail | 2025 |
| Ausome Growsome CIC | Agriculture & fishing | 2020 |
| Auspicious Shop Ltd | Wholesale & retail | 2023 |
| Austin Pharm Limited | Wholesale & retail | 2021 |
| Austin Software Engineering Limited | Information & communication | 2016 |
| Austin's School of Motoring Ltd | Education | 2022 |
| Austor Technology Company Limited | Wholesale & retail | 2025 |
| Autdors Sports Limited | Wholesale & retail | 2025 |
| Authentic Belfast Ltd | Hospitality | 2025 |
| Authentism Ltd | Health & social care | 2026 |
| Autism Consultancy & Training (ACT) Limited | Health & social care | 2008 |
| Autism Initiatives Northern Ireland | Health & social care | 2003 |
| Autism Ireland Limited | Health & social care | 2010 |
| Autism NI (Papa) | Health & social care | 2006 |
| Autism Ulster Limited | Health & social care | 2010 |
| Auto Apex Servicing and Repairs Ltd | Wholesale & retail | 2026 |
| Auto Benz NI Ltd | Wholesale & retail | 2020 |
| Auto Benz Trade Sales Ltd | Wholesale & retail | 2023 |
| Auto Clinic Automotive Ltd | Wholesale & retail | 2020 |
| Auto Collision Belfast Ltd | Wholesale & retail | 2021 |
| Auto Direct International Ltd | Wholesale & retail | 2019 |
| Auto Innovations (NI) Ltd | Wholesale & retail | 2020 |
| Auto Key Access Limited | Administrative & support | 2025 |
| Auto Pro X Ltd | Wholesale & retail | 2023 |
| Auto Pure Motors Ltd | Wholesale & retail | 2025 |
| Auto Q (I) Ltd | Wholesale & retail | 2007 |
| Auto Solutions N.Ireland Ltd | Wholesale & retail | 2026 |
| Auto Source Belfast Ltd | Wholesale & retail | 2021 |
| Autoglaze (2026) Ltd | Wholesale & retail | 2026 |
| Automated Intelligence Limited | Information & communication | 2010 |
| AUTOMATICS1 NI Ltd | Wholesale & retail | 2025 |
| Automation Controls (Belfast) Limited | Manufacturing | 1978 |
| Automotive Belfast Limited | Wholesale & retail | 2024 |
| Automotive Engines NI Ltd | Wholesale & retail | 2025 |
| Automotive Service Centre Limited | Manufacturing | 2017 |
| Automoveni Ltd | Transport & logistics | 2026 |
| Autonomie Ltd | Health & social care | 2023 |
| Autonomix AI Limited | Information & communication | 2025 |
| Autoparts Direct Holdings Limited | Financial services | 2022 |