"The issue she had . . . she kept it separate from her working environment," Sgt Ivan Howlin, of Dundrum Garda Station, told the inquest into her death at Dublin City Coroner's Court.
Her father Jim Ryan described his daughter as a "tremendous worker" despite turning to drugs at the age of 16 or 17.
The coroner's court heard how the young woman had tried to overcome her addiction to heroin at a rehabilitation centre in Holland and was on a methadone treatment programme at the time of her death.
Toxicology results presented at the inquest revealed the presence of cocaine, morphine and methadone in the young woman's blood when she collapsed in the early hours of August 11, 2008.
Ms Ryan, who worked as an administrator, was visiting friends at their home when they heard a loud bang coming from the bathroom. The friends rushed in and found Ms Ryan lying on the floor with a syringe beside her. Friend Rachel Kavanagh said Ms Ryan had a number of fits as she performed CPR on her friend after ringing emergency services. She was rushed to St James' Hospital, where she died on August 12.
Coroner Dr Brian Farrell said Ms Ryan appeared to have collapsed immediately after injecting cocaine.
He found that she died as a result of lack of oxygen to the brain due to cardio- respiratory arrest caused by cocaine toxicity. There was also evidence of recent use of morphine and methadone.
Niamh is survived by parents Jim and Vivienne and her brother James, aunt Elaine, and uncles Neville, Robert and Derek. The Ryan family declined to comment on the issue when contacted by the Herald. Vivienne Ryan stated: "We really do not want to talk about it".
Dr Farrell told members of Ms Ryan's family cocaine was a dangerous, unpredictable drug.
"Death can occur after injection of trivial amounts of the drug. It's totally unpredictable," he said.
"We see this unfortunately all too frequently at the coroner's court," he said."She was using the drug recreationally and it had a fatal effect."
He recorded a verdict of death by misadventure.