Animals are generally very good at avoiding poisonous plant material and usually only get poisoned when grazing is scarce or cut branches were thrown into a field.
I was planning on planting some common ash trees (Fraxinus excelsior) in a field, but I read on a webpage that they were poisonous to cattle and sheep. Is this true? Are there any of the native irish trees which are poisonous to cattle and sheep?
Ash may been implicated in some cases of poisoning, particularly where animals ate a lot of leaves that were green but fell after frost, and in the absence of proper grazing. But there are countless fields with ash trees on the boundaries and very rare occurrances of poisoning. Yew is a native tree that is poisonous.