How is the national electoral register created?

Department of Housing, Planning, Community and Local Government [website]

"What We Do
We are responsible for the legislation dealing with the registration of electors and the conduct of elections and referendums.

This involves:

- an ongoing review of electoral law
- review of constituencies and local electoral areas
- the provision of information and advice to registration authorities, returning officers and the general public together with
- the publication of election results. [...]"

"In order to be able to vote at an election or referendum, a person's name must be entered on the register of electors for the locality in which the person ordinarily resides. Registration authorities (county, city and city and county councils)  publish  the register of electors every year. The register comes into force on 15th February and is used at each election and referendum held in the succeeding 12 months." Source

Department of Housing, Planning, Community and Local Government is the electoral management body of Ireland. Voter register is created by local registration authorities (county, city and city and county councils), which function under the guidance of the Department.