How to get here? (The Witcher 3: Wild Hunt)

Go back to the cave where you went with Keira ... where she took the lamp nefig was to bring it down at the end of the task, since there is one more passage :)
I will not spoil it so that life does not seem like raspberries: D

YuraG
go further along the storyline, you will have a quest, and then and only then will you be able to open this dungeon.
Complete quests with Keira Metz

In the cave, when completing the quest, you examine

KamicazE said correctly, but if you finished the quests with Keira and got the magic lamp, then there will be no more quests in this cave. Now return to the cave and examine it well, there is nothing difficult, just go to the corridor where the lamp lay and see there will be another one the passage to the left you go along it and you will find everything.

YuraG
This place is underground. Can be found in the quest "Hunt for the Velma" http://www.playground.ru/wiki/witcher_3_wild_hunt/walkthrough/main_quests/velen/#wiki-29248

She is in the cave where you and Keira went

I suspected KamicazE ATP ))