Fitness groups in Phoenix Park are being charged up to €600 per year to train there by the Office of Public Works.
Boot camp and fitness trainers are outraged that they now have to pay €10 a day for the use of the public park.
The fitness gurus were able to use Phoenix Park free of charge up until last August, when a decision was made to charge commercially run groups.
The OPW has told the Herald that the fees must be paid in instances when the park is used for commercial purposes.
One anger trainer, Cathy O'Connor of Fitt Ladies Bootcamp, cannot afford to pay her fee of €610 because she is still trying to get her personal training business off the ground.
She told the Herald: "I lost my construction job two years ago and have since attempted to start my small business in the fitness industry.
"Last year the fees didn't exist; this year the OPW has advised they are looking for €610 paid upfront, and this is only from April to October."
Cathy holds outdoor group training sessions and one-to-one personal training in Phoenix Park, and she had hoped to increase her hours in the peak summer season.
"I started it last year and it's going okay, but I was banking on doing it from April to October.
"I can understand the permits and regulating where people are, but there aren't even toilets or drinking water. There are no facilities -- it's just a patch of grass."
Cathy, who is originally from Australia and she has been living in Ireland for the past six years, fears she will have to return home if she is charged the fees.
However, the OPW is adamant that fees are set at €10 per day, along with the usual conditions such as sending in a copy of public liability insurance and signing the Phoenix Park indemnity form.
An OPW spokesperson said the fees "came about when it was found that organisers of these groups were charging participants fees of €5.50 to €10 each to take part in each fitness training session".