What is the plural form for swordfish?

No, not an irregular plural. Like the noun fish, the noun sWordfish is both a count and a non-count noun, depending on use.

The noun fish (and sWordfish) is a non-count noun as a Word for a substance, a food.

The noun fish (and sWordfish) is a count noun for a creature living is the sea (or fresh water if applicable).

A regular plural is a noun that forms the plural by adding -s or -es to the end of the Word. An irregular plural is a noun that forms the plural in some other way (child, children; foot, feet; man, men; etc.)

The noun fish (and sWordfish) forms the plural, fishes (and sWordfishes) by adding -es to the end of the Word; a regular plural.

Example use for count and non-count:

Our group caught three sWordfishes.

We have enough sWordfish to feed the whole neighborhood.