Semple, John (1801–82), architect, was born in Dublin, the eldest son of John Semple (1763–1840) engineer and architect. As tradesmen, builders, engineers, and architects the Semple family formed a ubiquitous presence in Georgian Dublin. George Semple (qv) (d. 1782) was a renowned engineer who designed the St Patrick's cathedral spire, St Patrick's Hospital, and Essex Bridge; he was an advisor to the corporation of Dublin, and published treatises on The art of building in water and Hibernia's free trade. There was a John Semple in each of three successive generations of the family: John I (d. 1784), brother to George, John II (1763–1840), known as John Semple senior, and John III (1801–82), known as John Semple junior.
John Semple junior's grandfather worked on the Bluecoat School and the Customs House, while his father worked on Trinity College chapel and the King's Inns library. As a grand jury member, the latter benefited from public works contracts. He used the corporation guild structure to achieve patronage and advancement, becoming a sheriff's peer and member of the lower House of Sheriffs and Commons, and led ultra-right-wing corporate opposition to catholic emancipation. His certification of the Ha'penny Bridge suggests ability as an engineer, but his structural proposals for the Customs House stores led him into controversy with the engineer John Rennie (qv). With the construction of the Mansion House rotunda for the reception of George IV in 1821, and with his subsequent appointment as architect to the corporation, Semple's reputation was restored.
On his elevation to the archiepiscopal see of Dublin, and following his appointment as treasurer to the board of first fruits, the anti-catholic William Magee (qv) appointed a new architect to serve each of the four ecclesiastical provinces. John Semple junior was assigned to Magee's own ecclesiastical province of Dublin. During Magee's archiepiscopacy (1822–31) the practice of John Semple and Son produced at least twenty-five churches. These reflect a collaboration of architect and theologian in their references to early Irish Christian architecture, echoing Second Reformation episcopal protestant claims to patrician origin.
In the Blackchurch (St Mary's church, Mountjoy St., Dublin) a uniquely Irish corbelled vault recalls the ancient Glendalough church of St Kevin, while its catenary form suggests Magee's professorship of mathematics and Royal Irish Academy science committee membership. The simple churches at Grangegorman and Tipperkevin were followed by the more developed gothic revival churches at Whitechurch, Kilternan, Ballysax, Morristownbiller, and Donnybrook, all using the archetypal Semple formula of pseudo-stone arches, buttresses, string courses, and pointed doorway, features which recur at Clonygowan, Cloneyhurke, and Thomastown, and in the naves of St Selskar's and Mountmellick. Experiment in detail characterises Feighcullen, Tallaght, and Monkstown, while catenary theory informed the plaster vault of Rathmines and the corbelled stone vaults of Graigue, Abbeyleix, and the Blackchurch. With Magee's death Semple's work entered decline. Kinneagh, Kilanne, Redcross, Mountrath, and Taghadoe show little originality, while the stone arches at Inch parody the structural triumph of the Blackchurch. With the practice of John Semple and Son replaced at the board of first fruits, he languished as engineer to the piped water establishment until his ignominious dismissal by the reformed town council in 1842. As an architect he designed Carysfort Royal School at Macreddin, and added to Wexford and Carlow jails. Bankruptcy in 1849 saw the loss of his Regency villas at Seaview Terrace and Belgrave Square, and his death in 1882 ended 150 years of Semple ubiquity in Dublin.
Other earlier Semples include stuccodors Patrick and Edward (fl. 1760), and William (d. 1813), carpenter and celebrated treasurer of Dublin freemasons.