The Cathedral was designed by Fr Jeremiah Ryan Macaulay who had trained as
an architect before he became a priest of this Diocese. The original Cathedral of the Diocese of Down & Connor until the Reformation had been at Downpatrick and St Peter's was not originally intended to be a Cathedral when it opened amidst much celebration on October 14th 1866, this distinction was conferred on June 29th 1986 after having served as "Pro-Cathedral" for many years. The site for the building had been donated by the famous baker Bernard Hughes.
St Peter's is built of Scrabo sandstone with Scottish sandstone dressings. The stone-work has recently been completely restored, with new stone sourced from Scotland to replace that which had decayed. The restoration was directed by Maxwell-Pierce architects and was grant aided by the "Lottery Heritage Fund".
The Cathedral is a less ornate and smaller version of the great French medieval cathedrals. The exterior is quite simple in design, the most intricate detail being found on the twin spires. They are a majestic addition to the towers and are an outstanding feature of the Belfast skyline. The towers contain a carillon of nine bells. Above the west doors in the tympanum is a depiction of the "Liberation of St Peter" (Acts 12:1-11).
The overall dimensions of the Cathedral are 180ft (54.8m) by 70ft (21.3m). The interior is laid out in the normal ground plan of a nave and two aisles. The walls are supported by an arcade of seven arches with an eighth in the sanctuary. The east end terminates in a polygonal apse with five windows that feature simple geometric tracery. Internally the Cathedral has few original furnishings, having undergone several refurbishments in 1950, 1986 and 2003-2005. This most recent work has restored the Cathedral much nearer to its original appearance, especially in the use of strong and vibrant colours, on the fine hammer-beam ceiling. Detailed research has revealed the 19th century colours and stencil work inspired by A.W.N. Pugin, the celebrated Gothic Revival architect. The sanctuary ceiling is a different design, with coffered panels and arches that flank the windows below the I.H.S. monogram and angels carrying emblems of the Passion. The strong colours of the ceiling are repeated in the spandrels above the arches which contain pictures of Irish saints. These survived previous redecorations of the Cathedral and portray the Diocesan patrons, St Malachy and St MacNissi, and other saints associated with the Diocese, such as St Comgall and St Columbanus from Bangor Abbey, County Down, St Olcan and St Ibare. Also depicted are Saints Columba (secondary Patron of Ireland), Coleman, Asicus, Macartan, Finian and Finbarr.