The Health Products Regulatory Authority is investigating the first Irish case of a rare blood clot linked to the AstraZeneca vaccine.
The State's medicines watchdog said the the clot was detected in a person who had received the jab.
In a statement, the HPRA said: "The Health Products Regulatory Authority (HPRA) can confirm that it has received a case of interest, associated with the AZ vaccine, which describes CVST, an unusual clot in the brain.
"We are following up on this report to obtain additional details and to evaluate if it is consistent with the profile of rare blood clotting events that were the focus of the recently concluded EMA safety review," the statement continued.
"This report will be notified to the EU's safety database and will be considered in the context of continuous monitoring coordinated by the EMA.
"Due to confidentiality reasons, the HPRA will not be making any further comments on this case," it added.
The National Immunisation Advisory Committee (NIAC) will decide in the coming days if it needs to issue further advice to Government about the AstraZeneca vaccine.
It follows the finding by Europe's medicines regulator, the EMA, that there is a possible link between the vaccine and rare blood clotting in adults.
NIAC has said it will consult with EU colleagues and will consider over the coming days if it needs to issue further advice to the Department of Health in light of the decision by the European Medicines Agency (EMA).
At the moment it said its current advice remains that those receiving the vaccine should be warned that a small number of people who had recently received it reported very rare, complicated blood clotting events.
The Health Products Regulatory Authority (HPRA) has said the safety of the public was of the utmost importance.
The HPRA earlier said that the overall benefit of vaccination outweighed the risks of known side-effects, adding that it would continue to engage with NIAC and the department.
The vaccine manufactured by AstraZeneca makes up a fifth of the four million doses due to be received in Ireland over the next three months.
The Government has insisted it is still on target for 82% of adults to have received one dose of a Covid-19 vaccine by the end of June.
HPRA Chief Executive Dr Lorraine Nolan earlier said the public can be "really reassured" by the monitoring process of Covid-19 vaccines.
She said it is possible that the real world use may show up very rare effects that it is not possible to see within a clinical trial setting.
"We are doing it in a very open and a very transparent way, and I hope that the public are more reassured by that. That the process here is working extremely effectively," she added.
Instances of such rare clotting cases seem to be associated with the first dose of the AstraZeneca vaccine, according to a professor of immunology at Maynooth University.
Speaking on RTÉ's Morning Ireland, Professor Paul Moynagh said there have been 34 million doses administered, with 220 cases of rare clotting, and described it as a "rare event", which breaks down to a risk of it occurring in one of every 150,000 people.
He said countries are considering the use of another vaccine for the second dose, but there may be issues in terms of indemnity.
He said the AstraZeneca vaccine seems to affect younger people mostly, with women under the age of 50 appearing to be the main risk group, but it is "very rare and at these levels the EMA have decided the benefits clearly outweigh the risks.
He said Ireland has entrusted the EMA in terms of the licensing of the vaccines and it continues to recommend it across all age groups, but it is ultimately up to national regulators and NIAC is considering the latest issues that have arisen and will discuss it with their European counterparts later today.
A blood clotting specialist has also strongly urged people to take the vaccine. Consultant haematologist Professor Fionnuala Ní Ainle said while people should be aware of the warning signs of clotting post-vaccine, it is a very unlikely occurrence.
Prof Ní Ainle said that the type of rare clot identified effects the brain and abdomen, and involves two unusual types of thrombosis.
She said that there is a known link between Covid-19 and blood clots, and up to 20% of those who become most unwell from Covid-19 develop a dangerous blood clot.
She urged anyone who has received a vaccine to be aware of potential neurological symptoms including a persistent headache or a visual changes, as well as symptoms of lung and blood clots including chest pain, shortness of breath and leg pain or swelling.