Mountjoy can boast being Dublin's only true Georgian square, each of its sides being exactly 140 metres in length. While the North, East and West sides each have 18 houses, the South has 19, reflecting some variation in plot sizes. Though each side was originally numbered individually, the houses are now numbered continuously clockwise from no. 1 in the north-west corner. While its North and South sides are continuous from corner to corner, the East and West sides are in three terraces, interrupted by two side streets, Grenville Street and Gardiner Place to the West and Fitzgibbon and North Great Charles Street to the East. Gardiner Street passes through the West side of the square, while Belvedere Place and Gardiner Lane run off the North- and South-East corners.
Although some of the original buildings fell to ruin over the 20th century and were eventually demolished, the new infill buildings were fronted with reproduction façades, so each side of the square maintains its appearance as a consistent Georgian terrace.
Over the centuries, new features have been added to the street furniture while for various reasons others have been removed.
The street was originally paved with granite. By now, although the kerbs are generally still (probably Wicklow) granite, most of the paving stones have been replaced with concrete. The exception to this is the south-west corner in front of 53–54 which still has full granite paving.
Many of the houses on the square still have their original coal holes and ornate cast iron covers. These small holes in the street outside each house lead to a coal house underneath the street. These elegantly solved the problem of how to quickly and cleanly deliver coal to the house, allowing the coal men to simply pull the hole open and empty their sacks of coal down through it hole. The basement of the house then had a doorway into the coal house, under the street.