HEAnet provides a DNS service for HEAnet clients based on high availability primary and secondary nameservers. This service is ideal for small organisations which do not have the resources to operate nameservers and maintain DNS information themselves.
In essence, DNS is like a phonebook that links hostnames e.g. https://heanet.ie to a specific IP address.
When you open the page in your browser, it queries DNS for the IP address and matches the url https://heanet.ie to the appropriate IP address.