Semple, George (c.1700–82), architect, engineer, and building contractor, was born in Dublin, son of a workman; nothing more is known of his family background. Admitted a freeman of the city of Dublin (1735), he served an apprenticeship as a bricklayer; his earliest known work is a design for the 103 ft (31.4 m) granite spire of St Patrick's cathedral, Dublin, erected in 1749. The previous year Semple and Isaac Wills were each approached to compete to design and build a hospital for lunatics in Dublin, fulfilling the bequest of Jonathan Swift (qv). Semple delivered his design to the managing board of the hospital in November and was awarded the commission the following April, ahead of Wills. He began work on the hospital in 1749, fulfilling the requirement of Swift's bequest that the design should easily allow future enlargement. Construction of St Patrick's Hospital, as it became known (though often referred to colloquially as ‘Swift's hospital’), was completed in 1757; it resembled in some respects the Bedlam Hospital in London.
Semple acquired (1754) land on Queen St., Oxmantown, Dublin, building a number of houses there, and residing there for the remainder of his life. He built houses on Capel St. for – among others – Edward Leigh, treasurer of St Patrick's cathedral (1750–59); he also built (1749) Newbridge at Donabate, Co. Dublin, as a country residence for Charles Cobbe (qv), archbishop of Dublin. Semple also designed Headfort in Co. Meath (1760) for Thomas Taylour, 1st earl of Bective, and Ramsfort (1751), Gorey, Co. Wexford, for Col. Abel Ram. Ramsfort was destroyed during the 1798 rebellion.
Semple's most significant contribution arose from his boast (May 1751) to Cobbe that he could perform necessary repair work on Essex Bridge, Dublin, in less than ten days and for less than 100 guineas (£105). Indeed, such a tactic was common for Semple, who presented to various authorities, with varying success, unsolicited plans and projects throughout his career. Fulfilling this pledge the following month, Semple was then commissioned to reconstruct completely the bridge, which dated from 1676. As architect, engineer, and contractor, Semple demonstrated how these roles often intermingled; unusually, he also took on responsibility for the costing of the whole project.
The demands were far from elementary, due to the need to establish new foundations on the bed of the River Liffey. Semple consulted Labelye (the Swiss engineer then engaged in the construction of Westminster Bridge, London) and others on possible methods for laying new foundations. He approached (June 1752) George Ewing, a Dublin bookseller, to procure the latest literature on river construction; Semple had earlier spent £40 on books and plans in London, which proved to be of little use. Ewing's son and partner Alexander was in Paris at this time and returned with the latest French engineering works, including Forest de Bélidor's Architecture hydraulique, ou l'art de conduire (4 vols, Paris, 1737–53), which detailed the use and advantages of coffer dams. These enclosures were pumped dry of water, facilitating the building of solid foundations. The method, then in its infancy, compared to caissons – large enclosed sections of masonry sunk to the river bed – which Labelye used at Westminster. Semple adopted the ‘daring’ (Casey, 31) coffer-dam technique, the first time this method was used in Ireland.
The bridge, completed (1755) at a cost of £20,661, after two years and eighty days of uninterrupted construction, closely resembled Westminster Bridge in design, though it was arguably a greater technical accomplishment, with the removal of the old bridge and construction of new foundations directly onto bedrock. Widening it to 51 ft (15.5 m), much wider than the two streets approaching it, suggests that Semple was also the probable author of a plan (1757; NLI map 16 g 49 (6)) for the opening of Parliament St. southwards from the bridge onto a small square in front of Dublin castle. Indeed, the 1757 act commissioning a new street from the bridge to Dublin castle created the wide streets commission, which did so much to engender uniformity and expansiveness in the development of Dublin over the remainder of the century. On petition he was awarded £500 by the government in 1761; the pace and stress of the project took a considerable toll on Semple's health. The foundations remain in use, being retained when the superstructure of Semple's bridge was replaced (1873–4) by Grattan Bridge.
Semple described his methods, as well as the problems he encountered in constructing the bridge, in A treatise on building in water (Dublin, 1776). In the 1780 edition he added an exposition on the possible benefits to Ireland of a system of inland navigation, as well as a short autobiographical account, published as Hibernia's free trade (1780). In his work on both St Patrick's Hospital and Essex Bridge Semple gave detailed instructions for mixing cement and mortar, revealing his close attention to detail.
For the remainder of his life he seems to have occupied himself with promoting canal development. Semple lived in Queen St., Dublin, in 1763–7, 1769–75, and from 1776 till his death there in April 1782. His widowed daughter Elizabeth (details of his marriage, c.1740, are unknown) was the principal beneficiary of his will.