Native Irish trees.

I want to plant some native Irish trees in about half an acre of land to create a small dense wood. I have no plans to ever cut down these trees once sown.I was thinking about common ash (Fraxinus excelsior) and common alder (Alnus glutinosa) as an option. I dug a few holes in the field and on average there’s about 1.5-2 feet of depth in clay (good clay soil) but past that the ground is quiet rocky. So I was wondering if the trees that I am thinking about sowing are suitable for this area and how far apart these trees (or perhaps other trees) should be planted in order to let them grow to their full size. Have you a particular native irish tree that you’d like to see growing?