Are there any nouns with irregular plurals in Spanish?

Question

In English, some nouns have regular plural forms ending in -s or -es and fewer are irregular. Fish in the plural is still fish while child becomes children. In Spanish, nearly all nouns are regular, adding -s or -es. The rule is even simpler than in English (-es is for words that end in consonants and accented vowels other than *é*, s for all others). Also, words that end in *z* change the *z* to *c* in the plural to retain a pleasing sound in all regional accents.There are some words borrowed from other languages that have irregular plurals. In Mexico, the weekly markets are called *tianguis* (singular: *tianguis* plural: *tianguis*) after the Nahuatl word for the same weekly markets (often in the exact same places). *Tianguis* has been a common Spanish word since around A.D. 1520 but it still seems like a piece of traditional pre-hispanic Mexico.Are there any native or fully absorbed Spanish nouns with irregular plurals?

Answer 1

I wouldn't necessarily consider the noun *tianguis* to be irregular in the first place. Theoretically there are actually three, not two, morphemes to indicate plurality in Spanish. They are "-s", "-es", and ø, meaning the absence of a morpheme or morpheme zero.

The first type of plurality works for the word casa, meaning house in Spanish.

Singular- casa Plural- casas

The second is can be seen in the word pared, meaning wall.

Singular- pared Plural- paredes

The word lunes, meaning Monday, exhibits the third type.

Singular- lunes Plural- lunes

The third case is a special phenomena that mostly occurs in cases where the form of the noun already contains the same phonemes that indicate plurality. Scissors is a good example of this in Spanish and English. While the words scissors or tijeras appears to be plural already, it is a singular object. When forming the plural of this word in either language, nothing is added or changed making it fall into the third category, morpheme zero.

Answer 2

Compound words, such as lavaplatos is both singular el lavaplatos, and plural los lavaplatos.

-is endings: la crisis, las crisis

https://www.spanishdict.com/guide/spanish-plural-noun-forms has more irregulars.

Answer 3

To form the plural, and as a general rule, we add "s" or "es” according to the orthographic rules of the Real Academy of the Spanish Language. 

Some words make the plural both by adding an "s" and by adding the ending "es".

   Both are valid, but they are exceptions.