John Nelson Darby was born in London of an influential family. (His middle name is for the family friend Lord Nelson). At age 19 he was a gold medalist in classical studies at Trinity College in Dublin, and embraced Christianity during his studies. He joined an "inn of court" but felt that being a lawyer was inconsistent with his religious belief, and so chose ordination as an Anglican clergyman in Ireland. (There is no evidence that he studied theology). Darby travelled extensively ministering to the poor and ignorant of Ireland.
In October of 1827 Darby fell from a horse and was seriously injured. During his recuperation, he spent his time grappling with the issues of man's relationship with God. Darby decided that the Church comprised every true believer in Christ. He also came to believe that the very notion of a clergyman was an affront to the Holy Spirit working in individuals and the Church.
Within a year Darby had joined with others of similar belief (Dr. Cronin, Mr Bellett and Mr. Hutchinson) to "break bread" together in Dublin.
From this simple beginning "the brethren" arose. Over the next 175 years, there were many divisions resulting in "Plymouth Brethren", "Open Brethren", "Exclusive Brethren", "Kelly Brethren" etc. Each faction claims to hold "the truth" more exactly than the others, whereas in reality the factions are usually the result of human power struggles.
Being well educated and a forceful debater, Darby was able to dominate most of the brethren discussions in his time. This lead to him being considered the leader and, indeed, originator, of the brethren movement.
The brethren claimed to be teaching "rediscovered truths." Darby is noted in the theological world as the father of "dispensationalism." He is said to have originated the "secret rapture" theory wherein Christ will snatch away his true believers from this world without warning. Some authors (eg http://www.religion-online.org/cgi-bin/relsearchd.dll/showarticle?item_id=216 ) claim that dispensationalism theory influenced the British government to issue the Balfour Declaration 1917. If this is true, Darby influenced current world events.
Darby travelled widely in Europe and many "brethren" gatherings resulted. He also visited North America and Australasia.
He also used his classical skills to translate the Bible from the original texts. In English he wrote a Synopsis of the Bible and many other scholarly religious articles.
He wrote several hymns, the most famous being, "I shall be like thy Son."
Darby never married.
He died as the most respected elder of the brethren movement.