Although a photograph has been released to the press, the handshake between McGuinness and the British queen occurred behind closed doors. A second meeting was held a short time later, and the two shook hands again in front of television cameras for later broadcast.
Initial plans not to release a photograph of the handshake were widely criticized in Ireland.
At this second meeting, McGuinness spoke to the queen in Irish Gaelic, saying "slán agus beannacht," and explained it meant "goodbye and Godspeed."
Speaking to reporters a day in advance of the meeting, McGuinness said he was "still a republican."
McGuinness does not deny having been a member of the IRA, but claims he left the organization in 1974 in order to concentrate on politics as a member of Irish republican political party Sinn Féin. Few believe him. Across Ireland it is assumed McGuinness was a member of the now defunct IRA army council, the secret top brass of Irish republicanism, until it was finally wound up some time after 2005.
In another twist, McGuinness is alleged to have been IRA leader when it killed Lord Louis Mountbatten, the uncle of her husband.
Mr. Reid went on to say the "hugely significant step" was only one more in a process of reconciliation that "may take generations" to complete.