All public institutions must be able to receive and manage electronic invoices. The issuance of e-invoices by suppliers is voluntary.
e-Invoicing in Ireland
Characteristics of the electronic invoice in Ireland
Following the European directives, the electronic invoice is mandatory for public administrations and voluntary for public administration suppliers.
Irish Tax & Customs https://revenue.ie/en/Home.aspx
The national format is PEPPOL BIS. However, the public administrations have defined their own CIUS-CEFACT format.