The camera was spotted by a female chef who was taking a shower.
Whelan had moved it into the shower area from another location in a changing room where he had set it up six months earlier.
When gardai examined the camera they found only footage of Whelan installing the device.
Whelan admitted using the camera installed in the first location over the previous six months to record the 885 images and 30 videos of the women undressing and in their underwear.
The victims were all catering staff, aged between 28 and 56, working at the National Rehabilitation Hospital at Rochestown Avenue, Dun Laoghaire.
Whelan, of Farrenboley Park, Windy Arbour, pleaded guilty at Dublin Circuit Criminal Court to harassment of eight women at the National Rehabilitation Hospital on dates between July 21, 2011, and January 5, 2012.
Judge Martin Nolan suspended a jail term of four years after noting that Whelan is the significant carer for his elderly and sick parents.
Garda Sergeant Colm O'Giollain told John Fitzgerald, prosecuting, that Whelan was working as a contracted laundry worker and had access to the changing rooms used by catering staff at the hospital.
The court heard that Whelan had become obsessed with one woman and that he initially set the camera up in the changing room to get pictures of her. He denied that he was doing it for sexual gratification.
There were eight sets of folders found on his computer. Some of these contained videos ranging up to 24 minutes in length. One folder contained 416 images.
Typical folder names were: "Little bit of bra", "Experimental low shots" and "Milf".
The victims, who were previously on good terms with Whelan, said they felt betrayed and repulsed by his actions.
One of them was unable to socialise for six months and had made one attempt at suicide.
Whelan lost his job after the offences emerged and Caroline Biggs, defending, said he is now unemployable.
She said that Whelan was a single man who lived alone with his elderly parents and was the sole carer for his father who had Alzheimer's disease.
Counsel said that her client could not explain why he did what he did.