Pro Traveller is the Add-On that provides allowances for roaming usage. Full details, here: https://guides.plan.com/pro-traveller
Pro Europe covers International calling from the UK to Europe.
Pro Ireland covers calls and SMS (texts) from the UK to the Republic Of Ireland.
If you're unsure which Add-On is linked to your service, full details are listed on your customer agreement, which can be viewed from the dashboard of your my.plan account. Go to:
MY ACCOUNT > COMPANY DOCUMENTS > CONTRACTS