Hello Nathalie,
It's great to hear that you're planning to visit Cork!
To find green businesses, I have several options that come to mind.
You can have a look at sustainable accommodations available in the area. Quite often, these accommodations also offer the possibility of visiting their farm or even doing some work. You should be able to get in contact with them easily and plan a visit.
I would also suggest that you post this message on Ireland's Facebook groups. You might meet a local that would be happy to have you.
Finally, on Google Maps you can find some of the businesses and contact them on Whatsapp. Don't forget to add the Ireland country code when messaging, +353.
Is the school you're visiting "Clonakilty Agricultural College"? This college of agriculture might have some contacts and recommendations for you.
J'espère que ceci vous aidera et que votre voyage sera réussi ! Tenez-nous au courant !
Have a good day,
Marine.