Internally the building is dominated by the massive entrance rotunda with its elaborate City Corporation seal set into the floor mosaic. The motto of the corporation is the authoritarian 'Obedientia Civium Urbis Felicitas' which is roughly translated as 'Happy the city where citizens obey.' This is a later addition from when the Corporation took over the building. Originally the entire ground floor was one space divided only by the columns that support the dome but was subdivided to form office accomodation in the 19th century. . A contemporary report describes it:

The dome is spacious, lofty and noble, and is supported by twelve composite fluted columns, which, rising from the floor, form a circular walk in the centre of the ambulatory; the entablature over the columns is enriched in the most splendid manner, and above that are twelve elegant circular windows. The ceiling of the dome is decorated with stucco ornaments in the mosaic taste, divided into small hexagonal compartments and in the centre is a large window which lights most of the building.

The upper floor was accessed by two oval staircases at each corner of the north side of the interior. At the top of these stairs was a room used as a coffee room by the city merchants which is now the city Council Chamber. Unlike the Four Courts and the Custom House, the interiors of City Hall are intact as they were designed save for the alterations carried out in 1852. The subdivision of the ground floor has had the effect of emphasising the rotunda and its top lit dome. The restoration of 1999-2000 has removed these interior walls and restored the space to its original glory