Dudley Edwards was born and brought up in Dublin and educated at University College Dublin, Girton College, Cambridge and Wolfson College, Cambridge. Her father was the distinguished Irish historian Professor Robert Dudley Edwards. Her brother Owen Dudley Edwards is a historian at Edinburgh University. She married a fellow UCD graduate, the journalist Patrick Cosgrave in 1965 but they later divorced Obituary: Patrick Cosgrave | Independent, The (London) | Find Articles at BNET.com.

Although she has stated that she is "not in principle against Irish unification", Ms Dudley Edwards has generally taken a sympathetic approach to Unionists in Northern Ireland. This has caused her to attract considerable criticism and abuse from Irish People around the world, some of whom have alleged that she is not really Irish. Tom Hayden described her as "pro-Loyalist"

Her non-fiction books include An Atlas of Irish History, James Connolly, Victor Gollancz: A Biography (winner of the James Tait Black Memorial Prize), The Pursuit of Reason: The Economist 1843-1993, The Faithful Tribe: An Intimate Portrait of the Loyal Institutions (shortlisted for Channel 4/The House Politico’s Book of the Year) and Newspapermen: Hugh Cudlipp, Cecil King and the glory days of Fleet Street. Her Patrick Pearse: The Triumph of Failure (winner of the National University of Ireland Prize for Historical Research), first published in 1977, was reissued in 2006 by Irish Academic Press. In 2009 she published Aftermath: The Omagh Bombings and the Families' Pursuit of Justice a book about the civil case that was won on 8 June 2009 against the Omagh bombers.

Also a crime fiction writer, her novels include: Corridors of Death, The Saint Valentine's Day Murders, The English School of Murder, Clubbed to Death, Matricide at St. Martha's, Ten Lords A-Leaping, Murder in a Cathedrals, Publish and Be Murdered, Anglo-Irish Murders, Carnage on the Committee, and Murdering Americans.

Following the Cannes prize announcement, for The Wind That Shakes the Barley, Ruth Dudley Edwards wrote in the Daily Mail on 30 May 2006 that Loach's political viewpoint "requires the portrayal of the British as sadists and the Irish as romantic, idealistic resistance fighters who take to violence only because there is no other self-respecting course," and attacked his career in an article containing inaccuracies. The following week, Edwards continued her attack in The Guardian, admitting that her first article was written without seeing the film (which at that stage had only been shown at Cannes), and asserting that she would never see it "because I can't stand its sheer predictability."