Acting chief medical officer Dr Ronan Glynn last night appealed to Dubliners to follow Covid-19 rules.
"I am asking people everywhere, but Dublin and Donegal in particular, to pay special attention to the public health advice.
"I ask every individual to take personal responsibility to prioritise who you need to see, limit the size of your social network and reduce your social contacts over the coming days and weeks.
"Because while there is every chance that other areas will have to move to Level 3, there is nothing inevitable about it.
"We have seen previously how people working together can turn the tide on this virus and bring increasing trajectories back under control.
"Know how valuable your individual actions are. Your choices and your actions are part of how we will succeed."
Donegal joins Dublin in Level 3 today. Earlier, it emerged the country may be on course for locking down more cities across the country if the spike in Covid-19 is not brought under control.
Taoiseach Micheál Martin issued the warning amid ongoing concern about the spread of the virus in Cork, Limerick, Waterford and Galway.
Mr Martin said the Government will now consider city-wide lockdowns which spare rural county areas if recommended by the National Public Emergency Team (Nphet).
"The numbers are particularly growing in the cities and urbanised parts of those counties," he said.
"That will ultimately be a matter of Nphet as it advises Government. But our objective is to say to people living in these locations, we can avoid having to go to Level 3.
"We can keep the numbers down."
It comes as virus-hit Donegal began its first weekend of local restrictions as the chief medical officers here and in the North appealed to people not to make unnecessary journeys across the Border.
Dr Glynn and chief medical officer for Northern Ireland, Dr Michael McBride, met yesterday morning to review the ongoing Covid-19 pandemic.
Afterwards, they issued a joint statement which recommended that employers on both sides of the border make every effort to facilitate employees to work from home in so far as is possible.
"We realise that for those living in Border areas, this will not be welcome, but we must prevent further spread of this virus and we can only do so by working together to protect each other."
They noted specific concern with regard to the significant proportion of cases in young people in both Donegal and Derry, and appealed to teenagers and those in their twenties and thirties to reduce their social contacts.
It emerged yesterday that patients and staff on a ward at Regional Hospital Mullingar had to be tested for the virus.
The hospital shut down a ward to new admissions.
It comes as another 326 cases of Covid-19 were confirmed yesterday.
There were 152 cases are in Dublin, 32 in Cork, 22 in Donegal, 21 in Galway, 15 in Meath, 11 in Kildare, 9 in Kerry, 8 in Louth, 8 in Westmeath, 6 in Limerick, 6 in Mayo, 6 in Tipperary and 5 in Wexford, with the remaining 25 cases in 8 counties.
Higher Education Minister Simon Harris has issued a stark warning on the spread of Covid-19 saying the virus is "racing ahead of us at the moment".
He made the remarks as he announced new restrictions on colleges and universities.
Third level institutions are being asked to hold lectures online over the next three weeks.
Mr Harris, a former Health Minister, said that some towns and cities are "on a knife-edge"as he warned of the risk of the virus spreading.
He said he knows enough about the virus "to know that often we like to think we're in charge and the virus likes to show us who's boss.
"This is a virus that has gotten ahead of us again.
"It's racing ahead of us at the moment and we need to do everything humanly possible to get back in charge of this virus.
"I mean us as a people - all of us in terms of our own individual activities.
"There are things every single one of us can do this weekend to help."
He said there is simple advice from Dr Ronan Glynn, who has asked people to do two things: "Keep our distance and reduce our contacts.
"If each of us do those things it will help ensure we can continue to resume more and more contact and more and more priority services of which education is one of them."