Published Sunday, 24 May 2009
Nine players were shown the yellow card and Derry had captain Fergal Doherty sent off for a second bookable offence in the 63rd minute.
Sunday's game was punctuated by scenes of indiscipline and numerous off the ball incidents.
Cassidy said: "We aren't paid to entertain - this is an amateur sport and it's all about winning."
"I make no apologies for the way the game was played and I was proud of my players."
Paddy Bradley got the Derry goal in the first half and the Oak Leaf County survived having a man sent off, after Dohery was red carded in the 58th minute.
But they stepped up a gear and went on to score four points without reply through excellent scores from Joe Diver and Paddy Bradley which sealed victory.
In the semi-finals Derry will face Armagh or Tyrone on 21 June.
Paddy Bradley, Eoin Bradley, Barry McGoldrick and James Kielt all scored from play for Derry with Paul Finlay (two frees), Thomas Freeman and Rory Woods on target for Monaghan.
Derry 1-10 0-10 Monaghan