From The Irish Fireside, Volume 1, Number 10, September 3, 1883
Shortly before the year 1209, a pestilential disease which raged through Ireland had wasted Dublin. It was therefore deemed necessary to introduce a colony of English settlers from Bristol to repeople the city.
On the Monday in Easter week of that year the new inhabitants had proceeded, in a sportive manner to Cullen's Wood, some two or three miles distant from the city, to amuse themselves with rural pastimes during the holidays. But the mountain septs of the O'Beirnes the O'Tuathils (O'Tooles), who viewed the daily influx of foreigners into their native country with no friendly eye, lay in ambush for the unarmed Bristolians. At a convenient moment they suddenly sprang from their lurking place on the unsuspecting Britons, of whom they slew three hundred, besides a multitude of women and children who had accompanied their friends to partake in their harmless recreations.
The town was soon re-peopled by the citizens of Bristol, by whom the day on which their countrymen had been massacred, was denominated Black-Monday, an appellation which it has retained even to this present hour. During several centuries, the anniversary of this extraordinary event was solemnised by the mayor, sheriffs, and citizens of Dublin, in a very singular and curious manner. Tents were pitched upon the spot where the Bristolians had been slain. A joyous feast ensued, and, in the midst of the banquet, the imaginary enemy was publicly defied by mimic heralds, and warned, at his peril, to forbear from disturbing their revelry.
To this magnanimous challenge, thundered forth against the invisible O'Tooles and O'Beirnes, Echo alone replied. In process of time, the singing boys of the cathedral were deputed to utter this defiance ore rotundo, as if even they were fully competent to defend the city against so despicable a foe.
It is traditionally said that the Bristo-Dublinians, abhorring the O'Tuathils and the O'Beirnes, drove many individuals of those septs, innocent as well as guilty, from their native mountains. Some of the O'Tuathils settled near Lisnadill, in the neighbourhood of Armagh, where their posterity, I believe, resides at this hour. Others of the sept settled in Connaught, and in different parts of Ulster.
Such was the origin of Black Monday.
The term Bloody Tuesday seems to have originated with the Irish, who deem that day peculiarly fatal and unfortunate. Limerick was twice won, Wexford surrendered, Waterford besieged, and Dublin sacked upon a Tuesday!