The victim, Aisling Kelly, told a court she did not know who was calling and was left unable to sleep for fear that she was being watched and was about to be attacked.
Hendroff had phoned from a private number and spoke just once, while some of the calls featured "heavy breathing, panting and groaning", which Ms Kelly believed to be of a sexual nature.
Dublin District Court heard Ms Kelly, a solicitor at the office of the DPP, and Hendroff, a mountaineering author, became acquaintances through hillwalking. But when Ms Kelly ceased contact, he began making the calls.
Judge Catherine Murphy adjourned the case for sentencing after ordering the accused to stay away from the victim.
Hendroff, with an address at Moy Glas Drive, Griffeen Road, Lucan, pleaded guilty to harassing Ms Kelly on dates between February 6 and April 12 this year.
Sgt Sean McAvinchey told the court the first phone call to Ms Kelly's mobile lasted for 20-30 seconds and featured groaning.
The only instance in which Hendroff spoke was the "fail" call, just before Ms Kelly was about to take a test to be a team leader in mountaineering.
When her phone was analysed, there were more calls that went unanswered because Ms Kelly was afraid. Gardai traced the calls to Hendroff's phone, rang it and let Ms Kelly hear the voicemail message. She identified the accused from this.
Defence solicitor Peter Mullan said while the accused admitted making the calls, he did not accept that there was sexual content. The court heard his explanation that he made the "heavy breathing" calls "walking up a hill".
The court heard the defendant, a married man with no previous convictions, was "genuinely remorseful" for the hurt caused.
In a victim impact statement, Ms Kelly said she had met the accused as "another keen hillwalker" and had helped him with some Irish translations for his book.
He continued to make contact with her which made her "uncomfortable", and she told him to stop contacting her.
The anonymous calls then started and she could not sleep, fearing that the caller knew where she lived and that she was going to be attacked.
"I can't tell you how afraid this made me feel," she said. "When I found out it was Adrian Hendroff, I was angry but in some ways relieved that I knew who it was."
She said she was trying to forgive him for the "terrifying ordeal".