"It was an angry old woman" would be a perfectly correct English construction in answer to a question such as "Who was knocking at the door?" or "Who called on the phone?" or "Who threw a tomato at the politician?"

-- "It was an angry old woman."
-- "It was two teenage boys."

In the second example, notice that singular "it was" is used with the plural noun "boys" in the predicate. This is true even in a more complex sentence such as:

-- "It was two teenage boys who were throwing tomatoes."

The subject of the main clause is "it," (singular), but in the relative clause, the verb is plural ("were throwing"), to agree with "boys."

Compare this with:

[knock at door]
"Who's there?"
"It's me!"
or
"It's your neighbors, John and Mary!"