what does the bridge/tunnel? on the reverse of the 2006 pound coin commemorate?

Answer 1

The bridge is the Egyptian Arch Railway Bridge representing Northern Ireland. According to the Royal Mint the 2006 series of Reverse designs is required because the 4 year sequence of £1 coins has come to an end.The designs depict bridges from England,Scotland,Wales and Northern Ireland.

Answer 2

It's to represent Northern Ireland for 2006.

It's the MacNeill's Egypian Arch, which is a railway bridge in Northern Ireland.