The most important interior in the building was the House of Commons. Originally it was accessed via a doorway under the central portico through two ante chambers. Pearce's design (below in a contemporary painting of Grattan's Parliament) was octagonal with both a parliamentary chamber and a visitor's gallery delineated by a arcade of columns. The entire assembly by surmounted by a coffered pantheon like dome. Sadly this was all destroyed by fire in 1792.

The House of Lord occupied a less important position within the plan, off the main axis to the left. This however survives and is open to the public. It is accessed from Gandon's great portico to the east through a processional series of rooms and corridors. The corridors which wind around the original site of the House of Commons are lit with small top lit domes. A similar device is used to light the corridors of the State Rooms of Dublin Castle which were also designed by Pearce.

The Parliament consisted of two houses, the House of Commons and the House of Lords. The Commons is now the banking hall while the Lords remains intact. The most immediate impression is of the coffered ceiling and oak panelling. Two tapestries by John van Beaver depict the Battle of the Boyne. The remainder of the interior all date from the reconstruction into a Bank by Francis Johnston.