Northern Ireland Mercedes benz approved used

Have you enquired from the dealer on the car? Mercedes may have a policy of not allowing a Northern Ireland Mercedes dealer not sell a car to somebody from the south of Ireland.

Also is the car a Northern Ireland registered car or a mainland UK registered car imported into Northern Ireland by the dealer? This is common due to a wider choice of cars available in the UK. If it's a mainland UK registered car then there maybe VAT and customs duty due on top of VRT due to Brexit. The car would need to have been registered privately in Northern Ireland for at least 3 months to avoid VAT and customs duty afaik.

Does that not need to be under 3yrs old though and built in Europe, the merc would have been but it's over 3yrs.