Is this term an irregular noun wherein the singular form is also the plural form, or the plural form follows some odd rule?

A noun is considered irregular when

1. it is an uncountable mass noun
2. the term or phrase ends in a plural
   OR
3. the noun does not follow any of the regular noun pluralization rules

Noun pluralization rules

nouns that end in...

1. Most nouns add s to the end.
   cat – cats;
   car – cars;
   team – teams
2. ch, sh, s, x, or z  where es  is added to the end
   church – churches;
   tax ­­­­– taxes;
   pass – passes 
3. f or fe   where the f or fe becomes ves where  s is added to the end
   elf – elves; 
   loaf – loaves; 
   thief – thieves
   
4. vowel and y where s is added to the end
   toy – toys;
   boy – boys;
   employ – employs
5. vowel and o add s to the end
   video – videos;
   studio – studios;
   zoo – zoos
6. consonant and y where y becomes ies
   baby – babies;
   country – countries;
   spy – spies
7. consonant and o where es is added to the end
   hero – heroes;
   potato – potatoes;
   volcano – volcanoes