There had been a long tradition of iron vessels for use on canals and inland waterways. Sea-going iron steamships were pioneered by the Aaron Manby, built 1822 at London, for use on the Seine, with iron from the Horseley Ironworks, Tipton in Staffordshire.
  Another early iron paddle steamer was ordered by John Grantham (assistant to John Rennie in surveying the Shannon) for use on the Shannon. This was the iron paddle steamer Marquis of Wellesley (101 tons, 12 hp engine) - built in parts at Horseley and assembled at Liverpool from 1825 (under the supervision of Mr Page at Fawcett's), sailed to Dublin [so arguably the second sea-going iron steamer], and then reached the Shannon where she served from 1827. This vessel was unusual - two hulls with the paddle wheel between them.
  A pioneer of iron ship-building was John Grantham (Jr) who was the son of John Grantham who introduced steam navigation to the Shannon. He was Liverpool-based and wrote a definitive article (later book) "Iron as a material for ship construction" in 1842. He acted as a designer for many vessels built on the Mersey.
  He joined Mather, Dixon and Co. in Liverpool and later became a manager and partner in the firm. In 1843 Mather, Dixon and Grantham closed and Grantham began a practice as a Naval Architect and Consulting Engineer. He was involved in the design of several large iron sailing and steam ships, including Sarah Sands, Pacific, Antelope and Empress Eugenie.

Another of the pioneering iron sea-going vessels was the small paddle steamer Alburkah, (70 x 13 ft, draught 2.2 - 4.5 ft, 55 tons, 15hp, schooner rigged), designed by MacGregor Laird (son of William Laird, founder of the Birkenhead iron business, who had the vessel built locally) which voyaged to explore the shallow tributaries of the Niger River in 1832 and remained there for 6 years; the first ocean voyage by an iron steamship. Image from Das Heller-Magazin 1834.

William Laird and his son John developed their business from 1828 - moving from boiler making to ship-building, intially from a yard in North Birkenhead (on Wallasey Pool). They built several iron paddle steamers which were prefabricated: the Lady Lansdowne for the Shannon in 1834, the John Randolph sent to Savannah in 1834. They also built the small iron paddle steamer Garryowen for the lower Shannon Estuary in 1834 (the first sea-going iron vessel with watertight bulkheads). As well as naval gunboats, river vessels, and tugs, Lairds built passenger iron paddle steamers: Rainbow in 1837 was one of the largest. Engines were provided by Liverpool iron-works, especially by Fawcett & Co., and by Forrester & Co.

On the Clyde, the pioneering iron paddle steamer was the Fairy Queen built at Hamilton by John Neilson in 1831 for use on the inner Clyde.

Iron had advantages - lighter vessel, tougher, not susceptible to fire, repairable, higher scrap value; and disadvantages - not buoyant, no anti-fouling strategy, effect on ship's compass not initially understood.

Because iron vessels were lighter than wooden ones, they could be designed with shallower draught. Indeed, the first use of sea-going iron steamers on the west coast seems to have been to reach ports with shallow approaches - the quay at Glasgow, at Wexford and the lower Shannon.

Around 1839-1840; Tod & MacGregor (Glasgow) built the Royal George and Royal Sovereign for the Glasgow - Liverpool service; Grantham and Page(Liverpool) built the Brigand for the Wexford - Liverpool (and Bristol) service and Mather and Dixon(Liverpool) built the Erin-go-Bragh for the lower Shannon.

At Bristol the large iron screw steamer Great Britain was designed by Brunel, built by William Patterson and launched in 1843.