Just going on a trip through the nature of Ireland soon. Was curious as to what there is to eat in the nature? Any good fish in the river or some greens in the woods? I heared there is some nice seaweed at the atlantic coast.
A full discussion of forageable foods in Ireland would be a book, not a stackexchange answer. Fortunately, such books already exist. For foraging in the British Isles, one classic work (and a personal favourite of mine) is Richard Mabey's Food for Free (originally published 1972 and never out of print since). The introduction explains the book's remit:
This book covers the majority of wild plant food products which can be obtained in the British Isles. But there are some categories which I have deliberately omitted.
There is nothing on grasses and cereals. This is intended to be a practical book, and no one is going to spend their time hand-gathering enough wild seeds to make ﬂour.
I have touched brieﬂy on the traditional herbal uses of many plants where this is relevant or interesting. But I have included no plants purely on the grounds of their presumed therapeutic value. This is a book about food, not medicine.
This is also a book about wild plant foods, which is the simple reason (apart from personal qualms) why there is nothing about ﬁsh and wildfowl. But I have included shellﬁsh because from a picker’s eye view, they are more like plants than animals. They stay more or less in one place, and are gathered, not caught.
Apart from the omissions listed above, the book covers edible plants and fungi pretty comprehensively, including seaweed. It's available in a portable Collins Gem edition in case you want to take it with you.
Certainly there are fish in the rivers too, and other edible animals on land -- that would be another book-length answer. In this case I don't have a specific book to recommend, but the Fishing in Ireland website seems like a good place to start.