The Health Service Executive has said that those who are immunocompromised will be notified of an appointment for a third dose of Covid-19 vaccine from next week.
People will begin to be notified of their appointments from Wednesday and the administration of the third dose of the vaccine will begin on Friday.
Speaking on RTÉ's Morning Ireland, HSE CEO Paul Reid said contact would be made with people who are deemed at highest risk, saying that risk would be determined by clinical teams.
"It will be a period of five to six weeks to complete this programme," he said, adding that it will be complex with it focused on the most vulnerable and immunocompromised.
He said identifying the most at-risk would not be a simple process as it is not a list that they can "take off the shelf" but is rather a "complex piece of identification".
"It does address people who are highly immunocompromised, organ recipients, renal patients, certain cancer patients and certain people on certain medications," Mr Reid said.
He said if people are not contacted it is likely they are not in that highest risk category.
Mr Reid said the approach they are taking follows work with the National Immunisation Advisory Council (NIAC) who set out the early recommendations.
Mr Reid said the pathway for those who are entitled to the third vaccine will be clarified to them.
People will be directed to a vaccination centre. If they are in hospital, they will likely receive it there.
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Also on Morning Ireland, Director of Research with the Irish Cancer Society Dr Robert O'Connor said it was critical that those with cancer are afforded the best possible protection against Covid-19.
He said while they understand that a third dose of the vaccine is "imminent", he stated that part of the challenge is they do not have the data to know exactly how many people with cancer would need this additional dose.
He said it will take a while to identify the relevant cohort of people as people's treatment may be changing all the time.
Earlier this month, NIAC chair Karina Butler told an Oireachtas committee that patients with blood cancers and others who are immunocompromised will be included in the third dose roll-out.
Dr O'Connor said tens of thousands of people with cancer are immunocompromised, including those who are going through different types of cancer treatments and those living chronically with cancer such as blood cancers.
His advice is that those with cancer need to have conversations with their medical professionals to discuss this.
Oncologists would be keen to make sure there is maximum coverage for their patients, he said.
"Some people will be getting certain types of chemotherapy that will target parts of their immune system."
He also said that some people even with a third dose may not mount an effective vaccination.
Everyone can help to protect those who are immunocompromised, he said, by trying to keep community levels low by getting vaccinated, wearing masks and following the public health guidelines.