| Irish Tour Tickets Ltd | Administrative & support | 2015 |
| Irish Tourism (NI) Ltd | Administrative & support | 2017 |
| Irish Traders Ltd | Wholesale & retail | 2026 |
| Irish Voiceovers Ltd | Extraterritorial | 2024 |
| Irobust Investments Ltd | Real estate | 2025 |
| Irobust System PVT Limited | Information & communication | 2017 |
| Iron Donkey Bicycle Touring Limited | Administrative & support | 1998 |
| Iron Gecko Ltd | Information & communication | 2024 |
| Iron Horse Coatings (NI) Ltd | Manufacturing | 2024 |
| Iron Oak Form Work Ltd | Construction | 2026 |
| Iron Trades Employers Insurance Association Limited-The | | 1946 |
| IRP Commerce Limited | Information & communication | 2001 |
| IRS Energy Limited | Water & waste | 2012 |
| Irvine Brickworks Ltd | Construction | 2020 |
| Irvine Bros., Motors (Bangor,N.I.) Limited | Arts & recreation | 1973 |
| Irvine Film Ltd | Professional & scientific | 2022 |
| Irwin Court Property Management Company Ltd | Households | 2020 |
| Irwin Creative Limited | Professional & scientific | 2019 |
| Irwin Developments Limited | Construction | 2019 |
| Isaac Agnew (Holdings) Limited | Professional & scientific | 1931 |
| Isaac Lyons Holdings Limited | Real estate | 1967 |
| Isadore Properties Limited | | 1996 |
| ISC City Exchange Limited | Financial services | 2021 |
| ISD Consulting Engineers Limited | Financial services | 2023 |
| Isdm Solutions Limited | Information & communication | 2012 |
| Isensing Ltd | Information & communication | 2018 |
| Isherwood + Ellis LLP | | 2005 |
| Iskcon (Belfast) | Other services | 2006 |
| Islair Limited | Manufacturing | 2024 |
| Island Endeavour Limited | Financial services | 2020 |
| Island Events (NI) Ltd | Administrative & support | 2014 |
| Island Medical Media Services Limited | Health & social care | 2018 |
| Island Office Supplies Limited | Wholesale & retail | 1988 |
| Island Software Solutions Limited | Information & communication | 2016 |
| Islandmagee Energy Limited | Energy | 2006 |
| Ismarten Ltd | Other services | 2021 |
| Ismc Ltd | Administrative & support | 2024 |
| Isobel Lily Limited | Manufacturing | 2024 |
| Isoeco Ltd | Financial services | 2023 |
| Issac Arthur Ltd | Wholesale & retail | 2025 |
| Istay Property Management Ltd | Households | 2023 |
| Istmet Ltd | Professional & scientific | 2006 |
| IT & C LP | | 2019 |
| It's a Small World (Tully) Limited | Other services | 2017 |
| Italax.Net Limited | Information & communication | 2022 |
| Italian Food (Belfast) Limited | Hospitality | 2023 |
| Italy Gnam Limited | Wholesale & retail | 2022 |
| Italy Nihao Group Limited | Wholesale & retail | 2026 |
| Itera Properties Ltd | Real estate | 2012 |
| Ithaca Alpha (N.I.) Limited | Mining & quarrying | 2009 |
| Itni Ltd | Information & communication | 2014 |
| Itouch Smart Tech Limited | Information & communication | 2018 |
| ITS Computing Limited | Administrative & support | 1999 |
| Itson Group Ltd | Information & communication | 2024 |
| Itss Consulting Ltd | Professional & scientific | 2016 |
| Itss Limited | Information & communication | 2014 |
| Itty Bitty Book Co. Ltd | Wholesale & retail | 2019 |
| IU General Services Ltd | Information & communication | 2021 |
| Iustyimport Ltd | Wholesale & retail | 2023 |
| Ivan Gilmore Car Sales Limited | Wholesale & retail | 1984 |
| Ivanhoe Inn (Carryduff) Limited | Hospitality | 1980 |
| Iveagh Court Management Limited | Extraterritorial | 2003 |
| Iveagh Properties Limited | Real estate | 1999 |
| Iveagh Veterinary Services Limited | Professional & scientific | 2011 |
| Iverify Mobile Security Ltd | Information & communication | 2025 |
| Ivor Wilson Ltd | Wholesale & retail | 2023 |
| Ivotiddn Ltd | Wholesale & retail | 2025 |
| Ivywood Car Parks Limited | Real estate | 2008 |
| Ivywood Colleges Holdings Limited | Professional & scientific | 2008 |
| Ivywood Colleges Limited | Real estate | 2005 |
| Ivywood Colleges Parking Limited | Real estate | 2018 |
| IWC Convention | Other services | 2017 |
| Iwlen Slem Ltd | Wholesale & retail | 2024 |
| IYK Trading & Investment Company Ltd | Information & communication | 2023 |
| Iyrtiope Ltd | Wholesale & retail | 2024 |
| Izabela Dutka Aesthetics & Training Academy Limited | Other services | 2021 |
| Izer Group Ltd | Financial services | 2025 |
| Izer Trades Limited | Wholesale & retail | 2025 |
| Izon Live Limited | Information & communication | 2020 |
| Izzat Sports Limited | Arts & recreation | 2018 |
| J & a House Management Company Limited | Households | 2006 |
| J & C Frozen Foods Ltd | Wholesale & retail | 1998 |
| J & E Davy (UK) Limited | Financial services | 1994 |
| J & E Properties (N. IRE) Limited | Real estate | 2021 |
| J & F Magee Limited | Wholesale & retail | 2007 |
| J & G Construction (2010) Ltd | Construction | 2010 |
| J & I Adair Ltd | Real estate | 2024 |
| J & J Fleck Limited | Other services | 2011 |
| J & J Haslett Holdings Limited | Extraterritorial | 2003 |
| J & J Mowbray Limited | Manufacturing | 2012 |
| J & J Mowbray Properties Limited | Real estate | 2007 |
| J & J Property Belfast Ltd | Construction | 2017 |
| J & K Facades Ltd | Construction | 2025 |
| J & M Hospitality Limited | Hospitality | 2018 |
| J & N Finance Ltd | Financial services | 2025 |
| J & N Outdoors Ltd | Wholesale & retail | 2017 |
| J & S Hayes Ltd | Construction | 2015 |
| J & T Findlay Properties Ltd | Real estate | 2021 |
| J & W Body Repairs Ltd | Wholesale & retail | 2012 |
| J & W Distribution Limited | Wholesale & retail | 2016 |