I would guess that the old coach road follows a route from holyhead to Shrewsbury that may have been the basis for the A5. But usually A roads take a more direct route than old roads by adding bits of dual carriage way etc
Not the case here though. In 1801 the Act of Union with Ireland meant that Irish MPs needed a quick way of getting from their constituencies to Westminster. Telford got the job of building a road.
As far as Shrewsbury the coach road simply followed the old Roman Watling Street, but after that everything was built from new. At the time the cost/mile was equivalent to building a motorway today. But the quality of engineering meant that when it came to upgrading and the modern road classification system the A5 just followed the coach road. The only real changes since have been bypasses around Chirk and Oswestry. The rest is pretty much exactly as it was in 1839 when it was originally finished.
It's a great night leg though - easy to navigate, excellent surface and because it was built for horse drawn coaches, the gradient never exceeds 1:20 - perfect when you've got 300k in the legs.
The first year in 2007 we didn't have the control at Glyndyfrwdwy so there was a nearly 90k section along the A5 between Bangor and the next control at Chirk.