Over 2,600 Irish residents travelled to the North for private healthcare in the space of 18 months.
According to freedom of information figures, 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 eyecare.
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.