| Mcquillan Companies Limited | Real estate | 2024 |
| Mcquillan Developments Limited | Real estate | 1979 |
| Mcquillan Envirocare Limited | Water & waste | 1998 |
| Mcquillan Quarries Ltd | Mining & quarrying | 2015 |
| Mddsystems Limited | Professional & scientific | 2021 |
| Meadow Lane Property Ltd | Real estate | 2022 |
| Medfinancial Accountants Ltd | Professional & scientific | 2025 |
| Medical Collections Ltd | Administrative & support | 2023 |
| Medical Communications 2015 Limited | Information & communication | 2015 |
| Mediterranean Barber Lisburn Ltd | Other services | 2023 |
| Mediterranean Tanning Ltd | Other services | 2023 |
| Medlar Systems LP | | 2022 |
| Meenakeeran Wind Farm Ltd | Energy | 2022 |
| Meetco Ltd | Hospitality | 2026 |
| Meigenly Ltd | Wholesale & retail | 2026 |
| Melrose Media Ltd | Professional & scientific | 2020 |
| Men's Alliance (N.I.) | Health & social care | 2020 |
| Mercador Holdings Ltd | Financial services | 2025 |
| Merchant Commodities Ltd | Real estate | 2009 |
| Mercury Lease Co. Ltd | Extraterritorial | 2013 |
| Mercury Manned Guarding Solutions Ltd | Extraterritorial | 2013 |
| Mercury Security & Facilities Management Limited | Administrative & support | 2005 |
| Mercury Technology Solutions Ltd | Extraterritorial | 2013 |
| Meroë Foods Ltd | Manufacturing | 2020 |
| Merrfield Ltd | Professional & scientific | 2022 |
| Metaholdingsni Ltd | Real estate | 2026 |
| Metcollect Holdings Limited | Financial services | 2024 |
| Metcollect Limited | Water & waste | 2015 |
| Metro Hair Beauty Dunmurry Limited | Wholesale & retail | 2017 |
| Metro Hair Beauty Limited | Wholesale & retail | 2013 |
| Metro Hair Beauty Stranmillis Limited | Wholesale & retail | 2016 |
| Metro Lisburn Limited | Other services | 2025 |
| Metro MAN Dunmurry Limited | Wholesale & retail | 2017 |
| Metsamajand LP | | 2026 |
| MF Cooling Services (N.I.) Ltd | Manufacturing | 2018 |
| MGC Agencies Limited | Wholesale & retail | 2019 |
| MGM Orthopaedics Ltd | Health & social care | 2020 |
| Mhgg Studios Limited | Other services | 2018 |
| Michael COX Consultancy Ltd | Professional & scientific | 2023 |
| Michael Mooney Limited | Construction | 1949 |
| Michael Phillips Creative Ltd | Professional & scientific | 2024 |
| Michael Stewart Pressure Washers Ltd | Administrative & support | 2001 |
| Micon Distribution Limited | Wholesale & retail | 2002 |
| Mike & Sue's Convenience Stores Ltd | Wholesale & retail | 2007 |
| Mikson Ltd | Real estate | 2014 |
| Milagn Ltd | Wholesale & retail | 2024 |
| Milanos Ltd | Wholesale & retail | 2007 |
| Miller Management Group Limited | Financial services | 2017 |
| Milligan Hand and Upper Limb Clinic Ltd | Health & social care | 2025 |
| Missionary Homes (NI) Limited | Hospitality | 2004 |
| Mistley Bathrooms Limited | Extraterritorial | 2021 |
| Mite Limited | Real estate | 1996 |
| MJA (Broomhedge) Ltd | Real estate | 2024 |
| Mjsm Limited | Health & social care | 2018 |
| MLC Corporate Trustees Limited | Professional & scientific | 2011 |
| MLC Legal Limited | Professional & scientific | 2019 |
| MLC Legal Nominees Limited | Professional & scientific | 2011 |
| MLC Projects NO.50 Limited | Real estate | 2011 |
| MM 6638 Ltd | Hospitality | 2022 |
| MMD Decorators Ltd | Construction | 2017 |
| MML Electec Limited | Manufacturing | 2014 |
| MMZ Projects Ltd | Real estate | 2025 |
| MN Freedom LP | | 2021 |
| MO Marketing Group Ltd | Professional & scientific | 2023 |
| Mobile Shotblast Ltd | Administrative & support | 2019 |
| Mobiles Zone Ltd | Other services | 2020 |
| Modern Components Limited | Extraterritorial | 1987 |
| Modern Diamond Accomodations Ltd | Real estate | 2023 |
| Modern Machinery Supplies Limited | Wholesale & retail | 1966 |
| Moff Properties Ltd | Construction | 2011 |
| Moff Rentals Ltd | Real estate | 2025 |
| MOG Tools Ltd | Wholesale & retail | 2020 |
| Moira Engineering Machinery Ltd | Wholesale & retail | 2025 |
| Monadpay Ltd | Professional & scientific | 2025 |
| Monel Development LLP | | 2015 |
| Mono Bathrooms Ltd | Professional & scientific | 2019 |
| Monroe Consulting Ltd | Professional & scientific | 2025 |
| Montana & Belfort Consulting Firm LP | | 2025 |
| Monteloe Ltd | Real estate | 2019 |
| Montso Limited | Health & social care | 2019 |
| Moon & Back Tarot Limited | Arts & recreation | 2022 |
| Mooney Porter Holdings Ltd | Financial services | 2025 |
| Moore Robotic Mowers Ltd | Wholesale & retail | 2018 |
| Morebert Ltd | Wholesale & retail | 2024 |
| Morrow Gilchrist Associates Ltd | Professional & scientific | 2017 |
| Mosloz Trading Ltd | Manufacturing | 2023 |
| Moss Vale Limited | Wholesale & retail | 2024 |
| Mosscom Ltd | Professional & scientific | 2021 |
| Motexco Systems LLP | | 2018 |
| Motion R Design Ltd | Manufacturing | 2018 |
| MOTOCROSS4U Limited | Wholesale & retail | 2020 |
| Motorcycle Racing Association (Ireland) Limited | Arts & recreation | 1998 |
| Motorworx A.M Ltd | Wholesale & retail | 2023 |
| Motorworx RRR Ltd | Wholesale & retail | 2023 |
| Mount Eagles Square North East Ltd | Households | 2017 |
| Mount Eagles Square West Ltd | Households | 2017 |
| Mount Eden Properties Limited | Construction | 2011 |
| Mountain Pearl Coffee Ltd | Wholesale & retail | 2023 |
| Mourne Aerospace Limited | Professional & scientific | 2026 |
| Mourne Management Systems Limited | Professional & scientific | 2025 |