The right hand side of the Boyne Bridge (as you look towards the back of the bus yard) is dug out underneath so double deckers can pass underneath it. Occasionally there are lots of double deckers there; there are usually some playbuses and the wedding bus was kept there (correct me if I'm wrong) until somebody drove it into the bridge and it was scrapped.
I remember now hearing about the new map showing all the low bridges. There was an article in the Translink staff magazine as NIR helped with compiling it.