Are there any websites that sell gifts where you can plant a memorial tree in Southern Ireland?

There doesn't seem to be much available in Southern Ireland. Have linked to a site which can mail trees for you to plant and another which plants them, but you will have to contact them to find out if their Southern Ireland planting is up and running. You could contact your local council to find out if it is possible to plant a memorial tree on public land (such as a local park).