Thank you for reaching out with this question.
Like you, I do hope that your mother and her husband have a lovely trip to our shores and would be happy to help provide a bit of information to help make planning a bit easier.
Concerning needing a vaccination or recovery certificate to access indoor activities or hospitality, that was absolutely a travel requirement. As of January 22nd, however, this is no longer required for guests in the Republic of Ireland. There is currently no need to provide proof of vaccination or recovery to access local businesses, and you can learn more on the Citizen's Information
The situation is continuing to evolve, however, and there may be changes to travel requirements before their trip in April. What I'd recommend is keeping an eye on our traveler health page
to keep up to date with all the latest by following helpful links to the governments’ most recent guidelines.
I do hope this helps!