Is this an irregular plural? As I thought -au was the normal suffix for plural
welsh plurals a very irregular. This is an example of a word where the base word is the plural i.e Moron=Carrots and Moronen=(A)Carrot. These plurals are often found for animals, plants and other things that you're more likely to come across in groups Other Examples. Llygod=Mice Llygoden=(A)Mouse Plant=Children Plentyn=(A)Child Coed=Trees Coeden=(A)Tree and most tree types too Pinwydd=Pine/Fir trees Pinwydden=(A) Pine/fir tree Derw=Oak tree Derwen=(A)Oak tree. Also worth noting is that the -en ending is almost always feminine and -yn ending almost always masculine.
You simply have to learn it for most words though there are some endings which tend to be masculine or feminine. For example -en tends to signify a feminine noun so "moronen" is feminine. -yn is usually masculine so "plentyn" is masculine.