Since d --> n under nasal mutation, is the non nasal mutated form of nain dain or nain ? I suspect it is the latter.
nain is the root word, so no mutations apply.
When you need to check, try using Ap Geiriaduron (free) on a smart phone, or gweiadur.com on the web (needs free registration). Both will let you put in a mutated word or a conjugated verb and show you the root word. Gweiadur.com also shows you the full conjugated short forms of verbs, which can be very useful.