The green represents Irish Catholics, the Orange Irish Protestants, and the white peace between them.
The color green was traditionally associated with Ireland, and the revolutionary United Irishmen adopted a green flag as their symbol in the 1790s. A popular United Irishmen flag, deep green with a gold harp in the center, became the unofficial flag of Irish nationalism throughout the 19th century.
The United Irishmen and their immediate successors saw themselves as revolutionaries in the French mold and therefore tricolor cockades and rosettes came into use in the early 1800s–but in green/white/orange rather than French blue/white/red, with the orange intended to represent Irish Protestants (referring of course to William of Orange, whom Protestants revered as a champion of their liberties).
The first known use of the tricolor flag (as opposed to cockades) came in 1848 when it was introduced by the Young Ireland leader Thomas Francis Meagher as having been inspired by the flags of France and Newfoundland (where Meagher’s father had been born). Meagher failed to have the tricolor adopted as the new flag of Irish nationalism, however.*
The tricolor was unveiled by the 1916 rebels as the flag of the new Irish Republic proclaimed on Easter Monday of that year. It duly became the official flag of the new Irish Free State in 1922 and thus of course of the Republic of Ireland.
There is a story told that when Eamon De Valera escaped from an English prison in 1919 and went to the United States to drum up support for Irish independence, his followers proudly unveiled the tricolor to a packed hall. To their dismay, the display was greeted with jeers, catcalls, and shouts of “takedown that Eye-talian flag!” The Irish-Americans had been expecting the old and revered Irish nationalist standard–the United Irishmen’s deep green with the gold harp in the center.
For more detail, see: Flag of Ireland