So, this whole thing with nouns having so many different forms is an ever present stumbling block for me...

Can somebody shed any light on the genitive singular and the nominative plural being the same, whilst the genitive plural and the nominative singular is the same?? This seems extremely confusing and arbitrary.

Is there a reason that this feature of the language is a product of the natural evolution of the language?

Yes. The reason is the former case ending -i in genitive singular and nominative plural in Celtic (and Italic and other Indo-European languages).
Compare with Latin: vir - viri (man - man’s/men). It is absent in genitive plural: viri - vir(or)um (men - men’s).

The -i got lost in Irish but palatalisation of last consonants remains: fear - fir
Hence Irish fear - fir - fir - fear (man - man’s - men - men’s)