To understand Nisargadatta, you need to understand his teacher, Siddharameshwar such as found in Nisargadatta's book about his teachings called Master of Self-Realization.
Seeing the 'I' is just a concept is a Kintergarden awakening and not self-realization at all. Siddharameshwar and Nisargadatta (Consciousness and the Absolute) both put Self-Realization as the attaining and immersion in Turiya, also called by some the Love/bliss Body, or Krishna Consciousness (Nisargadatta Self-Knowledge and Self-Realization).
One gets there by following the sensation of "I Am," the "feeling" I Am down through the Subtle Body, the Causal Body of forgetfulness, down into the subtle essence of Consciousness itself in Turiya.
The is the dwelling place of Shakti and the manifest Self of bliss, knowledge and existence that Ramana talks about. This is where Ramana dwelled as did Nisargadatta most of his life.
But Siddharameshwar posited one further step: the Witness; the Absolute; the Unmanifest who was apart from Consciousness and which could never be known because it was beyond Consciousness. You could only be It, the Witness. During the last two years of his life Nisargadatta dwelt here. He was in pain, he was dying, and he identified more with the Witness, ParaBrahman.
Ramana did not posit a witness beyond Consciousness. If one were not aware of something, it did not exist. Consciousness was everything.
One can look at it this way: There are two Selves which are really one. The one Self of the Manifest universe was immersion in Turiya; on the other hand, there was the final Self of Nisargadatta and Siddharameshwar, identification with the Witness, being a witness. But you can say they are two sides of the same thing: the Self as Manifest and the Self as the Unmanifest.
But in neither case is Self the same as the conceptual self, the mind, the ego. That is left far behind during the first stages of either's path.
However, in a practical sense, they both shared the same Consciousness which is a primary identification with Turiya, but which also allowed them to identify with their bodies, emotions, pain, etc., as real life situations demanded.
My own teacher, Robert Adams, was ALWAYS in Turiya, and like Ramana, denied the need to create a Witness beyond Turiya. For him, Consciousness was self-contained, complete.