Your passports only intent and purpose is how you as a person are treated on your way to somewhere or if you are outside the BRITISH Isles.
Paddy, the area commonly referenced as the British Isles is Britain and IRELAND.
Very poor example, the french probably do not enjoy the same rights as a Vietnamese person, but you enjoy the same right's when in the UK as me.
Can I apply for a British passport as an Irish citizen living in Dublin who wishes to board a Ryanair flight to London?
A simple yes or no answer will suffice.
While you’re doing that google "French Indochina" and see what other countries made up French Indochina and then try to work out if they’re still the same country.