The Registry's policy is to follow the RFC1537, RFC1912 and RIPE DNS Working Group recommendations of 'minimal glue'.

A glue record will only be created for a host when it is nominated as an authoritative nameserver for a domain being delegated and its A resource record is to be found in the DNS tree in or below the zone that is being delegated.

A glue record is created for ns2.ucd.ie as it is specified as a nameserver for ucd.ie; but a glue record is not created for urda.heanet.ie as a nameserver for domainregistry.ie because that name can be resolved as normal, without glue (by forwarding the query to the heanet.ie authoritative nameservers).