What is the plural form for swordfish?

Answer 1

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.

Answer 2

The plural of swordfish is swordfish. Occasionally, swordfishes may be used, but the former is more correct.