The Black Dog, a debtors' prison that occupied this ground for well over a century, does not survive above ground in any form. By 1899 the Ordnance Survey was already annotating the spot as simply "Black Dog (Site of)", marking a levelled vacancy between two medieval city gates, Gormond's Gate and the New Gate, along the line of the old Dublin City Wall. What makes the place linger in the historical imagination is less its architecture than the paper record of what went on inside it.

The prison appears in the Dublin Assembly Roll as early as 1576, when a John Fitzsymons was formally charged with the keeping of prisoners committed to what was then called the New Hall. By the time John Gilbert was writing in 1854, he noted that the site had also contained the New Hall Market, with upwards of eighty butcher's stalls crowding the same ground. The most detailed account of the prison's workings, however, comes from a 1730 parliamentary inquiry into conditions there and at his Majesty's Gaol of Newgate, both of which were, at that point, effectively run by a single man, John Hawkins, who had held the combined offices of Keeper of Newgate and Sheriffs-Marshal since an Act of Assembly dated 9 May 1721. The inquiry found that every prisoner arriving at the Black Dog, even for a single night, was charged 2s. 2d. as a so-called "Penny-Pot" fee, and that refusal to pay was met with beatings and the stripping of clothing. Convicted transportees were deliberately mixed in with debtors to enforce the levy. Those who could not meet the weekly rent of 12d. per person per night, collected personally by Mrs. Hawkins, were dragged to an underground room known as the Nunnery: roughly 4 metres square, 2 metres high, unboarded until shortly before the inquiry, with no light except what filtered through a drainage sewer running alongside it. The inquiry recorded that up to twenty people were crowded into this space in a single night. John Perry, a cutler aged over sixty-five, spent six days and nights on the floor of that dungeon after merely mentioning the possibility of a habeas corpus removal; he was carried out unable to walk. Mary Campbell, an elderly widow, arrived to find her daughter obliged to pawn a silver spoon to cover the Penny-Pot fee, and was given a sleeping space beneath a staircase for 3s. 6d. a week.

There is nothing to see on the site today in any conventional sense. Upper Bridge Street runs through what was the prison's footprint, and the medieval wall line it once sat against has itself largely disappeared. Rocque's 1756 map of Dublin city, widely reproduced and available to consult in digital form, shows the building clearly labelled on the western side of the street, which gives some sense of its position relative to the surviving fabric of the old city nearby. The area repays a slow walk with a good historical map in hand: the proximity to the old gate sites means that the geography of the medieval city is still faintly legible in the street pattern, even if the Black Dog itself left nothing behind but its name and a very thorough set of committee minutes.