Richard Morrison was the architect of the original building, described in 1824 in the Provincial Directory of Pigot & Co., as "a modern stone building". It was extended towards the front by John McCurdy circa 1860 and there were further improvements by County Surveyor John Brett a decade later.
In 1997, Deaton Lysaght Architects undertook extensive renovations which ensure that the building will continue to function as a courthouse for many decades to come. Morrison’s building stood well back from the pavement on a site, the property of Thomas Burgh, which had been chosen in 1797 for a Sessions House to replace the decayed 17th Century building further up the main street. Morrison’s courthouse had a double-height hall, with four free standing columns, flanked by two parallel courts, and an open-well staircase lit by a skylight. In the basement, there were cells for male and female prisoners, and two water closets, with an entry from outside the building. A narrow passageway from the rear of the courthouse led to the canal harbour and it also gave access to the new gaol when it was built in 1833.
In McCurdy’s improvements, the courthouse was extended towards the front, and a portico erected, perhaps that from the earlier building, with steps leading down to a granite paved forecourt, enclosed with curved railings on both sides. The courthouse was restored following damage by fire in the 1950s and in 1997 it was extensively refurbished and enlarged to accommodate four courtrooms. Imposing features of the building are the foyer and the spiral staircase with its decorative railings of brass and wrought iron, lit from an oval sky-light. The panelled Victorian double-height courtroom, which has featured in many films and television programmes, was then also re-modelled. The original canopy and galleries remain, but the well (with its panelling) was removed to storage for possible reconstruction in a museum.