Donnybrook was the location for the Donnybrook Fair. This event was notorious for drunkenness and violent disorder. The fair was banned in 1855.
So, today the word donnybrook is applied to a quarrel fight or disturbance usually involving disorderly and violent behavior.
The above definition is the most popular one to explain why a fight is sometimes
called a donnybrook. Others say the word origin is from a battle because of the
Battle of Donnybrook. In this fight, the Scots repelled the British.