The number of patients waiting for operations or medical procedures is significantly higher than the waiting lists published by the National Treatment Purchase Fund, which has responsibility for collating waiting list data.

RTÉ Investigates uncovered evidence that shows there are at least two other significant waiting lists which are not published by the NTPF, which have tens of thousands of patients awaiting treatment.

The Pre-Admit list comprises of 22,927 patients from across the country waiting up to 18 months on surgical procedures. These numbers are not included in the monthly reports published by the NTPF.

The second list called the Pre-Planned List contains the names of 59,843 patients who require follow up treatment ranging from surgical-pin removal, to cataract removal, to hip replacement and even corrective spinal surgery.

Patients on the pre-planned list are typically given indicative dates for their procedures. These dates can range from less than six weeks to more than 12 months.

Speaking to RTÉ's News at One, HSE Interim Director of Acute Hospitals Liam Woods clarified that the list also includes over 34,000 endoscopies where patients had initial treatment and require further scopes at regular intervals in the future.

When the endoscopy figure is discounted, the numbers on the unpublished lists come to almost 49,000. These figures are not included in the published lists.

Speaking on RTÉ's Six One, Mr Woods said the HSE is not contesting today's figures, adding that 81,000 waiting for a service is too many.

He said the visibility of the data on its waiting lists is entirely appropriate, but so too are the categories.

Mr Woods also said that more investment is needed in the health service.

Liam Woods, HSE says the underlying challenge for the health service is to the reduce waiting times for service pic.twitter.com/ZFgnhH2HO9 — RTÉ News (@rtenews) February 6, 2017

There have been calls for greater transparency in the way the NTPF reports the data.

Former minister for state and sitting Louth Fine Gael TD Fergus O'Dowd has strongly criticised the NTPF saying the waiting list figures "are being massaged" and the public have a right to full access to this information".

In a statement, the NTPF said "in line with international best practices, published waiting list data excludes patients classified as Pre-Admit and Pre-Planned Procedures" … the NTPF is currently "examining updated international best practice around publication models".

Taoiseach Enda Kenny has said he understands the exceptional difficulties that some people have on hospital waiting lists.

Asked about the RTÉ investigation into waiting lists, Mr Kenny said he looked forward to seeing the programme tonight. He said no one wanted to see anybody in difficulty or waiting in pain and he said the matter was a priority for Minister for Health Simon Harris.

In a statement this evening, Mr Harris said: "The programme clearly illustrates the physical and emotional burden of waiting for long periods for both patients and their families.

"I am keenly aware of this burden and it is for this reason that last summer I requested that the HSE put in place an Action Plan to halve the number of patients waiting over 18 months for treatment.

"Last month I announced the successful completion of this Action Plan with over 11,500 patients coming off the Inpatient/ Daycase waiting list in the second half of 2016...In 2017 I am determined that the HSE builds on last year's progress and reduce waiting time for our longest waiters."

Speaking on RTÉ's Claire Byrne Live, the minister said that he would work to reduce the waiting lists.

He said: "To those of you who are still waiting, I want you to know that I am going to work might and mane to make sure that by the end of this year, we have dramatically reduced the length of time patients wait in this country."

It's not fair for a child to go through this when it can be rectified with surgery. She doesn't have to be like this.

The HSE has issued a statement saying it "apologises to any patient who has had a poor experience either in terms of waiting times or communications with our services".

The statement continued: "It is unacceptable that any patient in our care, particularly patients suffering and in pain should experience any difficulty in making contact with our hospitals.

"A key priority for the HSE is tackling long patient waiting times and ensuring timely access to treatment and care.

"We fully acknowledge that patient waiting times are unacceptably long in certain specialities and this, to some extent, reflects the need for greater capacity and staff requirements within our services."

The HSE said it is "aiming to ensure that the end of June this year no one will be waiting over 18 months. We are also aiming to ensure that 85% of cases are seen within 15 months."

Megan Halvey Ryan, 13, from Limerick, suffers from scoliosis and is featured on tonight's "Living on the List" broadcast. Her spinal curvature has worsened dramatically in recent times. She now finds it almost impossible to attend school.

Living on the List: Megan has a curved spine and the pain is causing her to miss school #RTEInvestigates pic.twitter.com/YYt35jHLDr — RTÉ News (@rtenews) February 6, 2017

"Megan just cannot manage the pain. She's missing school. It's affecting her whole life and as a consequence it's affecting the whole family life," says her mother Sharon Halvey Ryan.

"I just want my surgery, it's not fair," said Megan.

"It isn't fair. It's not fair for a child to go through this when it can be rectified with surgery. She doesn't have to be like this," said Ms Halvey Ryan.

"She needs her surgery and [then] this will go away. Her life can start again. And she's not having her surgery, we don't have a date. We're still in limbo," she added.

Meanwhile, Betty Rogers, from Offaly, has a painful and debilitating spinal problem. Her quality of life has drastically diminished as she struggles to cope with the pain. She has been on various waiting lists for surgery on her spine for almost two years.

"I do be in there sometimes in the kitchen and I'd be leaning on the sink. I just have to lean on it to take the pressure off my back with the pain. And then I'd have to come in and sit down," says Ms Rogers.

"Being on a public list is just...the thing is endless. You see no light at the end of the tunnel," says her husband Pat.

Watch RTÉ Investigates - Living on a List