Recently these started to annoy me, so I contacted the email for cleansing queries on the Belfast City Council website. They said to try the Department for Infrastructure. I contacted them and they said motorways were managed by a private company, Intertoll. I contacted them and they said cleaning the walls isn’t in their remit.
Quite an eyesore for people entering the city, though Wikipedia tells me that me and the Westlink are very close in age, so this may be some kind of mid-life crisis.
Anyway, here’s a grim gallery. For free, I’ve thrown in one of those ultra-dreary bridges from up the road, and a brutal M1 central reservation.
David Mitchell 
At least you’ve got a motorway over in Belfast even if the bridges look a bit grim. People are dying over this side of the country for lack of a proper dual carriageways – I mean the A5!