I have seen walnut growing by a road in monmouthshire and whinberries on top of drystone walls in the same county. Hedgerows frequently have trees left to grow to maturity.Not too sure about most fungi I have seen stinking hellebore in hedgerows and mushrooms in the verge. I would agree quiet byways are the places to go hunting.
anyway for your sojourn in Ireland:

www.irishexaminer.com/lifestyle/foodanddrink/darinaallen/edible-wild-plants-found-along-the-roadside-are-added-to-all-the-following-recipes-399485.html