Question about Brexit: Eventually, will I need a work permit/visa to visit and work in Ireland as a British citizen.?

Brexit has zero to do with the UK/ROI freedom of movement. Northern Ireland has no intention of "joining" the Republic, there are no plans for a hard border again, good job as it would bring the roads to a standstill in rush hour as thousands of Irish work and travel daily to NI or travel into NI to do their weekly shopping, so my guess is ( and it is only a guess) as the CCTV cameras are already in place since the hard border finished the ANPR system could be used for all registered British and ROI citizens vehicles and you have to use plane/ferry to get onto the island so immigration/customs checks will be stepped up in both the North and Republic

Nothing different is happening regarding Ireland. we have always been able to work in the whole of Ireland without a Visa.
UK