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iasgfishmasculine noun
the
an t-iasg
after a preposition
an iasg
of the
an èisg
plural
èisg
the (plural)
na h-èisg

Every form but the headword, the genitive and the plural is worked out by the spelling rules in the grammar guide, so what you see here is what the guide teaches. Awaiting a fluent speaker’s review.