"It is a fruit."
Translation:Is toradh é.
You're not wrong, but an and na are definite articles - "the" in English (unlike other European languages, English uses the same definite article for singular and plural).
There is no "the" in "It is a fruit", so an and na don't come into play.
"It is a fruit" - Is toradh é
"They are fruits" - Is torthaí iad