AntonyMMM that makes sense with the dashes - cheers
https://en.wikipedia.org/wiki/General_Register_Office_for_England_and_Wales#History_of_the_GRO"from 1 July 1837. A General Register Office was set up in London and the office of Registrar General was established. England and Wales were divided into 619 registration districts (623 from 1851), each under the supervision of a Superintendent Registrar.
The districts were based on the recently introduced poor law unions. The registration districts were further divided into sub-districts (there could be two or more), each under the charge of Registrars who were appointed locally.
A similar process was utilised in Ireland when Civil Registration commenced there in 1865 following from official gov.ie website with slight editing so as not to confuse matters:
In the early days, the organisation of the service was based on the "unions" of parishes set up under the Poor Law (Ireland) Act 1838. Initially, these unions formed the registrars' districts and later the superintendent registrars' districts [when births & deaths started 1864]. The Clerk of the Union was usually the superintendent registrar. The medical officer of the dispensary district was usually also the registrar of births & deaths.The term Union can be seen at the top of all the 19C births & deaths in the GRO book. It makes sense that they copied the existing proceedure utilised in England.
Sorry need to scroll across to right of image. Effectively it says births in the Sub-District of Lurgan in the District of Lurgan = Union.
[The medical officer may not have acted as the Register in England think that was partly as they were percieved as politically/religiously neutral].