The entrance to Barnard’s Inn (Wikipedia), a small inn of court, which was on the south side of Holborn.
The overlays that highlight 17th century London features are approximate and derived from Wenceslaus Hollar’s maps:
Open location in Google Maps: 51.517786, -0.109759
Barnard's Inn is south off Holbourn (sic) west of Fetter Lane
Chart showing the number of references in each month of the diary’s entries.