Ignatius Loyola Donnelly (November 3, 1831 – January 1, 1901) was a U.S. Congressman, populist writer and amateur scientist, known primarily now for his theories concerning Atlantis, Catastrophism (especially the idea of an ancient impact event affecting ancient civilizations), and Shakespearean authorship, all of which modern historians consider to be pseudoscience and pseudohistory. Brother to Eleanor C. Donnelly (Wikipedia page ), Donnelly's work had important influence on the writings of late 19th and early 20th century figures such as Helena Blavatsky, Rudolf Steiner, and James Churchward and has more recently influenced writer Graham Hancock. Donnelly's concept of Atlantis as an antediluvian civilization became the inspiration for the 1969 pop song hit Atlantis by Donovan and the 2009 film 2012 by Roland Emmerich.
In 1882, Donnelly published Atlantis: The Antediluvian World, his best known work. It details theories concerning the mythical lost continent of Atlantis. The book sold well, and is widely credited with initiating the theme of Atlantis as an antediluvian civilization that became such a feature of popular literature during the 20th century, and contributed to the emergence of Mayanism. Donnelly suggested that Atlantis, whose story was told by Plato in the dialogues of Timaeus and Critias, had been destroyed during the same event remembered in the Bible as the Great Flood. He cited research on the ancient Maya civilization by Charles Étienne Brasseur de Bourbourg and Augustus Le Plongeon, claiming that it had been the place of a common origin of ancient civilizations in Africa (especially ancient Egypt), Europe, and the Americas). He also thought that it had been the original home of an Aryan race whose red-haired, blue-eyed descendants could be found in Ireland.
A year after Atlantis, he published Ragnarok: The Age of Fire and Gravel, in which he expounded his belief that the Flood (as well as the destruction of Atlantis and the extinction of the mammoth) had been brought about by the near-collision of the earth with a massive comet. This book also sold well, and both books seem to have had an important influence on the development of Immanuel Velikovsky's controversial ideas half a century later.
In 1888, he published The Great Cryptogram in which he proposed that Shakespeare's plays had been written by Francis Bacon, an idea that was popular during the late 19th and early 20th century. He then travelled to England to arrange the English publication of his book by Sampson Low, speaking at the Oxford (and Cambridge) Union, where his thesis "Resolved, that the works of William Shakespeare were composed by Francis Bacon" was put to an unsuccessful vote. The book was a complete failure and Donnelly was discredited.
More information on the Wikipedia page