Aren't the adjectives supposed to agree in case, gender and number? Why isn't it "na súile beaga"?
That would be true if "small" was the attribute of the subject, like if you wanted to say "His small eyes are blue", but it's not. Here "the eyes" are the subject of the sentence, while the predicate is "are small". Since the adjective is not directly attached to the noun, it doesn't have to agree with it.