Mr Bradshaw says ‘..Pentonville …Of late years it has extended rapidly, Barnsbury Road and Barnsbury Park, a new and daily improving district, derive their names from Lady Juliana Berners, Abbess of St Albans, who had a large manor is this neighbourhood..’.
However, The London Encyclopaedia says the name derives from Bernersbury Manor which was held by Ralph de Berners, who came to England with the Norman conquest, and subsequently his family, to 1536. Whatever the origin of the name, there was once a manor in this area, which was probably moated. Barnsbury Square was the centre of the Bishop Estate which developed from 1834 onwards. In the early 1800s William Tufnell, Lord of the Manor, with an eye to London’s expansion and need for housing, began leasing land to developers, including Nathaniel Bishop. (Barnsbury Square was previously known as Pond Field, or Reed Moat Field.) The Bishop Estate was built by Thomas Whowell who lived in Mountford Crescent. The square has an odd shape, with Mountfort Terrace and Mountfort Crescent at the west end, and there is no architectural uniformity, as in the other squares in Islington.
Mountfort House dates from 1836 and was built as two residences and dominates the square. Multiple builders, and WWII damage, have produced a square quite unlike any other in Islington.
The gardens in the square are beautiful and were particularly gorgeous in the spring, as were some of the gardens in houses around the square. I hardly noticed the buildings!
You may also be interested in A History of Islington Squares A Walk through Islington