Nope, "Maneg"(glove) just undergoes a vowel change to form the plural to become "Menig". (Maneg is feminine by the way.) A good way of knowing if something becomes singular by adding en/yn is to think if it is commonly found in groups of more than two (also a lot of things in nature).
There are about 30-40 different ways of forming plurals in Welsh. There are a few recognisable patterns, but they all have exceptions. The best thing is to look up and learn the plurals as you meet new nouns. Ap Geiriaduron is a good free smart-phone app which shows plurals, and www.gweiadur.com is a good on-line dictionary (free registration).
When first starting to learn Welsh there is not much need for the plurals, or for the singulars of some items as EllisV points out, but once you move on a bit you suddenly realise that you do need to know them!