Upvote:3
The UK's guidance on the British-Irish Visa Scheme has this to say (about people who aren't eligible for BIVS):
If a national of a country requires a visa for the UK but not for Ireland, those nationals will still require a visit visa to travel to the UK. The same will apply to any nationals who are not required to have a visa to visit the UK but are required to have one for Ireland.
This describes the current situation in law.
So far only nationals of India and China can receive BIVS visas. Everyone else must apply for the visas separately as described there.