Over 2,600 Irish residents travelled to the North for private healthcare in the space of 18 months.
According to freedom of information figures released to the Today FM newsroom, this cost the HSE over €9 million.
The Northern Ireland Planned Healthcare Scheme was introduced at the start of last year.
It allows Irish residents to get private healthcare in the North, and be reimbursed by the HSE.
The most common treatments people get are orthopaedics and eye care.
Just under 1,200 people availed of the scheme last year, costing the HSE just over €4 million.
That increased to over 1,400 patients in the first six months of this year, costing almost €5 million.
In the same 18-month period, over 2,400 travelled to the EU under a similar scheme, called the Cross Border Directive.
This cost the HSE nearly €9.9 million.