Is removing a diesel particulate filter legal in Northern Ireland?

No for normal road use where the vehicle was designed with one. DVA specifically identifies physical removal of a DPF or associated sensors as improper modification of emissions-control equipment.

For this Northern Ireland query, the practical issue is whether removing a diesel particulate filter legal in Northern Ireland. Northern Ireland uses DVA vehicle-testing standards, so a modification is judged by what it does to the vehicle rather than by whether it is fashionable or sold online. The finished vehicle must remain safe, correctly constructed, roadworthy and compliant with lighting, emissions, tyres, steering, suspension, restraint and noise requirements that apply to it. DVA's modification guidance says manufacturer-fitted emissions-control equipment must be maintained and not removed, bypassed or tampered with. It names catalytic converters, DPFs, sensors, EGR/SCR systems and mapping changes that defeat emissions controls as examples of improper modification.

Before spending money, check the DVA inspection item that the modification affects and tell the insurer exactly what has changed. Keep invoices, part specifications and approval information. A part being sold as “MOT friendly” or “road legal” is not proof that a particular installation on your car complies; poor fitting can turn an otherwise acceptable component into a defect. For this question specifically — “Is removing a diesel particulate filter legal in Northern Ireland?” — the safest answer is the one above, based on the current Northern Ireland and UK official guidance rather than forum advice or rules written for another jurisdiction.