The Linger singer head-butted the cop as she was being arrested following an alleged 'air rage' incident on an Aer Lingus plane bound for New York in November, 2014.
O'Riordan was escorted off the flight before take-off and police officers apprehended her in the airport terminal, where she informed them she was "the Queen of Limerick".
She was diagnosed as suffering from bipolar disorder after the incident, and medical reports have since suggested she was mentally ill when the incident took place.
At Ennis District Court in County Clare on Wednesday (24Feb16), a judge refused to convict O'Riordan, but told her she would have to pay a fine to charity.
The Irish singer pleaded guilty to assault charges in December (15). She was also accused of attacking three policemen who had been called to remove her from the aircraft and was charged with three counts of assault and one of wilful obstruction of police.