You can edit the destination of your Irish VoIP purchased DID in your client area in irishvoip.com. Enter the SIP Address (IP or hostname) and SIP port to which you wish to DID calls to be delivered.
Calls are delivered using SIP UDP.
See below to configure your DID in Asterisk or Freepbx
https://www.irishvoip.com/index.php/knowledgebase/104/AsteriskorFreepbx-Settings-for-DIDs.html
Additionally, see below for details for Firewall Whitelisting
https://www.irishvoip.com/index.php/knowledgebase/139/Firewall-Rules-for-DIDs.html