The vaccine booster campaign will begin as early as next week.
The HSE has confirmed the third doses of the Covid jab will be given to vulnerable groups.
HSE boss Paul Reid said those eligible for a third vaccine dose can make appointments from Wednesday, with the first doses given next Friday.
However, Mr Reid said the process of identifying those eligible for the third dose was complex.
There was not a “simple list to take off the shelf,” he told RTÉ’s Morning Ireland.
Mr Reid said that the campaign would take five to six weeks. The HSE will contact those they consider “high risk” to make appointments. Clinical teams will identify those in this category, he explained.
The vaccines will be administered through vaccination centres, hospitals, care settings and through GPs, he said.