Patients will now need to meet revised criteria to qualify for a COVID-19 test, including displaying two major symptoms of the virus.
It comes amid increasing demand for coronavirus testing in Ireland, with around 18,000 tests carried out to date.
Patients must now have a fever and at least one respiratory symptoms (such as a cough or shortness of breath) to be referred for testing, after Ireland adopted the World Health Organisation case definition.
Contacts of a confirmed case, healthcare staff and vulnerable groups will be prioritised for testing.
Thousands of people waiting for a COVID-19 test who don't have an appointment yet will now not be tested and will have to contact their GP again.
Health officials are warning that people not in a priority group may not be tested.
However, anyone with symptoms is still being told to assume they have COVID-19 and to isolate themselves for 14 days to help stop the spread of the disease.
Household contacts of a suspected case should also restrict their contacts for two weeks.
Dr Tony Holohan, the Department of Health's Chief Medical Officer, says they have to prioritise cases due to the number of people seeking a test.
He explained: "With the significant increase in testing that we've had, still only 6% of those are confirmed cases.
"That has been a challenge - to continue to reach all the people who would seek a test, and to get those tests to people in the timeframes that they would like."
The Department last night said contact tracing has shown the average number of close contacts of confirmed cases has decreased to around five people - down from more than 20 - due to social distancing measures and advice.
Analysis of public health contact tracing has shown that the average number of close contacts per confirmed case has decreased from 20+ to the region of 5 contacts. This shows that the public is following health advice and actively limiting the amount of people they engage with.
— Department of Health (@roinnslainte) March 24, 2020
It comes as significant new restrictions come into force from today, with non-essential retail outlets told to close temporarily.
Last night it was announced that 204 new cases of coronavirus have been confirmed in the Republic, bringing the total number of cases to 1,329.
A total of seven people have now died after being diagnosed with COVID-19 here.