Are words that stay the same irregular plurals?

No, words that stay the same are not irregular plurals. They are called zero plurals or null plurals.

Irregular plurals are words that change their form in the plural, often in unpredictable ways. Examples include:

* Child - Children

* Foot - Feet

* Mouse - Mice

Zero plurals are words that have the same form in both singular and plural. Examples include:

* Sheep - Sheep

* Series - Series

* Species - Species

So, while both irregular plurals and zero plurals deviate from the regular "-s" plural formation, they are distinct categories.