Brennan, Louis (1852–1932), engineer and inventor, was born 28 February 1852 in Castlebar, Co. Mayo, youngest of three sons of Thomas Brennan, grocer, ironmonger, and hardware merchant, and Bridget Brennan (née McDonnell). His eldest brother, Michael George Brennan (d. 1871), was a notable journalist, caricaturist, and artist. In 1861 the family emigrated to Melbourne, Australia. Educated at Eade's technical college in Melbourne, and articled to A. K. Smith, CE, Brennan displayed a striking aptitude for mechanical engineering. In 1874, eight years after Robert Whitehead had devised the first self-propelled torpedo, Brennan set about developing a dirigible torpedo. Australian state, naval, and academic backing carried the project through to initial tests in Australia (March 1879). Moving to London (1880), he secured agreement by February 1883 from the admiralty to bear the cost of research and further trials till 1886 and to provide an annual retainer in return for his supervision. He fulfilled the terms of the contract within the time allotted, but it eventually took skilful negotiation to sell the patent rights to the admiralty in January 1887. Faster, more accurate, and with greater range and explosive capacity than Whitehead's torpedo, Brennan's was suitable primarily for use in fixed harbour defences and not in submarines. Till 1896 Brennan was superintendent of the torpedo factory in Gillingham, Kent (in production 1887–1906), when the weapon was the basis of most UK shore defence systems. He moved to ‘Woodlands’, Gillingham (1892). He was made CB (1892) and awarded honorary membership of the Institute of Royal Engineers (1896), in recognition of his contribution to military engineering.
Taking up part-time consultancy with the war office in 1896, in order to work out the concept of the gyroscopically stabilised monorail, he poured the bulk of his personal fortune into this project between then and 1909. Despite making great advances in gyroscope technology, and causing a media sensation (1906–7) with well-flagged and successful tests of a working model, he failed to attract funding from the army council and encountered resistance from commercial railway companies, and indeed there were practical disadvantages to the system. He was, however, awarded principal prize at the Japan–British exhibition at White City in September 1910. The sale of ‘Woodlands’ in 1912, and later full-time employment with the Ministry of Munitions, steadied his household finances. Called on (1916) to bring to efficient working order his long-nurtured designs for a helicopter, Brennan made use of scanty resources over the next decade to construct a machine capable of vertical ascent and hover, but fiscal prudence on the part of the treasury put a stop (July 1926) to his appointment, and to a project which predated in all significant respects the achievement of Sikorsky in 1939. Undaunted by this disappointment, Brennan passed the next few years perfecting a design for a gyrocar. While recovering from sickness in Switzerland from December 1931, he was knocked down by a car and died in hospital on 17 January 1932.
He married (10 September 1892) Anna Mary (d. 24 April 1931), daughter of Michael Quinn of Castlebar. They had three children. Resembling in character and in scientific passion some of the more humane technical visionaries in the early novels of H. G. Wells (his name appears in the first chapter of The war in the air (1909)), Brennan had an imagination teeming with inventive design: some thirty-eight of his ideas were patented, but many were not. The National Museum of Science and Industry in London maintains a model of his monorail car. Gillingham library, Kent, holds a substantial mass of private papers and other material relating to his life and work.