This is a lovely quiet and green spot set back behind a set of office and apartment buildings. It used to be a vegetable garden (huerto) for the cloistered nuns that lived in a the Convent del Sacramento nearby in the 17th Century. In the 1970s the immediate area was redeveloped but the garden space was kept. It seems that someone has started a tiny veggie garden on the top of the stairs going towards Calle del Rollo but otherwise there is a lot of ivy ground cover and some fruit trees. Sadly there isn't any information available on site on its history.
There are 2 entrances to the garden: Calle del Sacramento, 7 & Calle del Rollo. If you come in from Calle del Rollo, its on the bend, walk under the glass window office bridge. If coming from Calle del Sacramento, walk through the open gate down the stairs that lead below the apartments. Either way you'll see greenery ahead. Note that the google info is wrong, this garden space is only open Mondays - Fridays, 8am - 6pm. It is sometimes called Jardín del Palacio O'Reilly.
It is close to a similar small 'hidden' garden 'El Jardín del Príncipe de Anglona' but this one seems to have much fewer visitors.