Ms Monahan has worked with Dunnes for 30 years and was manager in the lingerie and hosiery department.
Her claim includes €135,000 for loss of earnings: she only returned to work in April 2011.
Micheal P O'Higgins said the accident occurred after olive oil had spilled on the floor.
Ms Monahan had been in storeroom and was on the way to talk to the store manager when she slipped on the liquid.
Dunnes was claiming she should have been aware of the oil as employees were trying to mop up and had cordoned off the area.
Ms Monahan, who fractured her shoulder, claims she was not prevented from approaching by the manager .
Counsel told Ms Justice Mary Irvine that water had been used in the mop-up and this had made the floor more slippy.
She went home and later had x-rays. She went into work the next day, but had to go home because of the pain.
She had to have two operations and intensive physiotherapy, but will have permanent pain in her shoulder as a result of her injuries, her counsel said.
A medical report of 2010 said there was reduced range of movement in her shoulder.
She had had to give up her philosophy degree studies and can't lift heavy loads and or hang clothes out on the line, counsel said.
The case resumes on Tuesday.