500 years after the Romans' departure, I don't think roman weaponry would be much seen. Some weaponry could have a roman ancestry, yes, such as the Anglo-saxon ridge helmets (Coppergate, Wollaston) had. Penannular broochs were also of a far roman ancestry but there is a huge difference in style between a simple roman penannular brooch and an 8th century Irish one.
Irish had a long-time influence in Wales. Anglo-saxon were more and more influents, and of course the Norse had a huge impact on weaponry and fighting styles in the whole British isles by that time. I think it would be wise if you have lot of anglo-saxon or norse weaponry (especially swords and helmets) equipping Welsh elite.
For some more 'regional' stuff you can go for (and I may be wrong):
- perhaps still exiting, basic ridge helmets with or without cheekguards
- smaller round shields, eventually leathercovered that would equip lot of the troops
- 'roman derived armor', simply mail or scale armor with specific shape including some pteruges (leather bands) from the padded garnment worn underneath. Not sure if this was still around but it often appears in carolingian art althought often dissmissed by scholars as artistical convention.
The Welsh could fashion longer tunics, and some could go barelegged in the Irish fashion.