Is strongly correct in the following, or should it be strong?
... and had a strongly Protestant and unionist identity.
What is the explanation in grammar terms?
Both are correct, but strong is a bit better.
The difference is subtle. I think both statements are accurate, but the former is closer to what the article means to say. So strong is the right word to use.
Martha says that strong Protestant and unionist identity is strictly speaking incorrect unless you add commas. I disagree. It’s completely standard. It is easy to find plenty of examples of this pattern in good journalism and academic writing. Here are some examples (found in the Corpus of Contemporary American English):
There are thousands more. The pattern also appears in the name of the National Oceanic and Atmospheric Administration, a U.S. government agency.
"Strongly" is okay in this sentence. To help clarify:
"Strongly" is modifying "Protestant and unionist" to compare it against a mildly Protestant/union identity. Using "strong" would change the meaning and require some fiddling:
Here, we could remove the last two adjectives:
The possible source of confusion is the "and" between "Protestant" and "unionist":
This probably sounds less awkward.
Strongly is correct. In grammatical terms, strongly is an adverb modifying, in this case, the adjectives Protestant and unionist, which are in turn modifying the noun identity.
There is a tendency in modern English to use adjectives as adverbs, resulting in things like "... a strong Protestant and unionist identity...", but strictly speaking this is incorrect, unless you add commas to make the adjective modify the noun instead: "... a strong, Protestant, and unionist identity..." — but obviously the latter means something quite different than the version without commas.
That said, unless you're writing for a particularly nitpicky audience, either choice is fine and will be understood as an adverb (unless marked as an adjective).