Cancer patients, diabetics and newborns in neonatal units are believed to be on the "critical list" being compiled to guard against medicine shortages here after Brexit.
The main concern is that a disorderly Brexit will mean a delay of containers with drug supplies at customs.
Ireland's medicines watchdog, the Health Products Regulatory Authority (HPRA), has been working at various levels for two years to prepare for a worst-case scenario.
Although a list of critical medicines has yet to be finalised, the greatest risk is to a small number of products, including radioisotopes which have a short shelf-life and are used in radiotherapy for cancer patients.
Other medicines which could be affected are those which require cold storage. These include insulin, antibiotics and products like eye drops. The other drugs which are being given special consideration are those which need specialised manufacturing processes. Some products mentioned include total parenteral nutrition, which is specially formulated for sick or premature infants and imported from the UK.
Suppliers are being contacted to ensure there are contingency plans in place and the Revenue Commissioners are being asked to ensure customs delays are minimised.
Although patients are being told not to stockpile, suppliers are doing "bridging stock" which allows for eight to 10 weeks of drugs to be stored.
The HPRA has an existing system in place to deal with regular drugs shortages and there can be 45 notifications a month.
The preparations have also involved the big drug companies who supply branded medicine.
A spokesman for the large companies body, the Irish Pharmaceutical Healthcare Association, said: "Given the vital nature of our business to human health, the industry has been preparing for a 'no deal' Brexit.
"Our primary goal has been to protect patients.
"The industry is investing heavily in ensuring that we are prepared, as best we can, for every eventuality.
"That means reorganising supply chains and revising regulatory approvals so that delays getting medicines to patients are avoided."
Almost 70pc of medicines supplied in Ireland come to us from or through the UK.
This has meant drug giants re-organising how drugs are brought to Ireland and bypassing the UK in some cases.
Darragh O'Loughlin of the Irish Pharmacy Union, representing pharmacists, said they are waiting for the full critical list of medicines.
He said a lot of work has gone into preparations.
"A problem could arise where there is just one single source of supply. There may be just one company making the medicine. In the case of generic drugs, there are likely to be several manufacturers," he pointed out.
He also expressed concern about delays to trucks carrying medicines.
Other measures to help supply include working with the UK drug regulator to maintain joint labelling of products.