who discovered it?
A Portuguese explorer, Tristão da Cunha.
Tristan da Cunha
The islands were first recorded as sighted in 1506 by Portuguese explorer Tristão da Cunha, though rough seas prevented a landing. 
where is the most isolated island on earth Tristan da Cunha, is in the south Atlantic Ocean, lying approximately off the coast of Cape Town in South Africa, from Saint Helena and off the coast of the Falkland Islands.
wiki:19591976
Tristan da Cunha [SEP] Discovery
The islands were first recorded as sighted in 1506 by Portuguese explorer Tristão da Cunha, though rough seas prevented a landing. He named the main island after himself, "Ilha de Tristão da Cunha." It was later anglicised from its earliest mention on British Admiralty charts to Tristan da Cunha Island. Some sources state that the Portuguese made the first landing in 1520, when the "Lás Rafael" captained by Ruy Vaz Pereira called at Tristan for water. The first undisputed landing was made on 7 February 1643 by the crew of the Dutch East India Company ship "Heemstede," captained by Claes Gerritsz Bierenbroodspot.