Apex domains cannot have CNAME records.
Question and response
Question
dig moodle-plesk1.heanet.ie. NS
Answer
moodle-plesk1.heanet.ie. 5 NS ns1.moodle-plesk1.heanet.ie.
moodle-plesk1.heanet.ie. 5 NS ns2.moodle-plesk1.heanet.ie.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0