Visa for Northern Irish Sponsor

[kake]

I'm applying for fiance visa for UK to marry to my fiance from Northern Ireland.
My fiance was born in Northern Ireland, he was born after 1984 and only holds Irish passport. (His parents were both born in Northern Ireland, and they only hold Irish passports)
I'm wondering if he is allowed to call himself a British citizen/has Nationality for United Kingdom.

I thought the circumstance of your birth make you British, and his birth certificate proves his British nationality (even though he doesn't hold a British passport)
He only holds Irish passport, does it mean he is only Irish citizen, not British citizen even though he and his parent's born in Northern Ireland and live there permanently?

For UKfiance visa application form, I write his Irish passport number, his nationality is Irish, and state he holds another nationality which is British and he is a British citizen.
Do you think I must provide additional information stating He is British citizen and British nationality since he was born in Belfast? Is birth certificate not good enough evidence?

I am not going for EEA route, I can only apply for UK route.
Your help will be GREATLY appreciated !!!