Just for info - a bit of a curate's egg, this one: it looks like this changeset fixed one (part of a number affected by a different change by someone else last week) and broke two:

11a12
>  -15480473 | Killiney DED 1971
126d126
<  -13061512 | Killiney ED 1901
169d168
<  -12498334 | Dalkey ED
1132c1131
< (1129 rows)
---
> (1128 rows)

Relations 13061512 12498334 still fail in Josm's validator.

I was in contact with the person who made the original change at the weekend, and they said they'd try and have a go at fixing things this week.

Of course, it could be argued that perhaps the best place for "historic electoral districts from 1901" might be OHM rather than OSM, but I couldn't possibly comment on that :)