McDonald feeling the pressure after voters turn backs on Sinn Fein in Republic of Ireland
Sinn Fein is facing dismal election results in the Republic, with dozens of losses projected across local councils.
There is also doubt over whether the party will be able to hold on to its three outgoing Irish MEPs, with Dublin candidate Lynn Boylan having a nervous wait.
The party is expected to suffer council losses in Dublin city, as well as Cork city, Galway, Limerick, Offaly and Cavan.
Among the electoral casualties yesterday was Belfast-born Sarah Holland, who failed to retake her seat on South Dublin County Council.
She was first elected in 2014, and served as Mayor of South Dublin in 2015.
Ms Holland is the daughter of greengrocer Harry Holland (65), who was murdered in Belfast in 2007.
Sinn Fein deputy leader Michelle O'Neill expressed her commiserations to Ms Holland, tweeting: "Disappointed for you Sarah. Keep your head up, you will be back."
An exit poll commissioned by RTE forecasts that Sinn Fein's vote share in the local government election could be 12%.
That would represent a drop of three percentage points from the 15% support it won at the local government election in 2014.
Dublin TD Louise O'Reilly said she did not see the drop in vote share coming.
She shied away from analysing the developing situation yesterday morning, saying it was something the party would examine in detail afterwards.
She told RTE: "We are going to have to analyse what went wrong.
"It's a difficult day (for the candidates)."
However, the party is likely to be buoyed by the expected election of one of its new candidates, former MMA fighter Paddy Holohan on South Dublin County Council.
Last night, Sinn Fein's Lynn Boylan said she still believed she had a "fighting chance" of taking a seat in the Dublin constituency.
Arriving at the count centre in the Simmoncourts with her party leader Mary Lou McDonald, Ms Boylan said lots of speculation was going on - but they would wait for the actual results to be tallied.
Ms McDonald added: "We are here to see what will transpire, we know this is a tight race.
"Everybody knew this was going to go down to the wire from the beginning.
"What we have is exit poll data, so we are here to get the real figures.
"We come here in a spirit of optimism and a spirit of hope."
On Saturday, Ms McDonald said she expected tight contests in many constituencies, with final seats likely to come down to handfuls of votes.
"That's the nature of local elections," she said. "I am not exactly sure where we are going to land, we will have a lot of counting and a lot of long nights ahead of us."
However, Ms McDonald was defiant when asked whether her leadership was at risk.
She said: "It's easy to lead and to be a political activist when things go your way and when the surge is on. Those are great days, but you also have to be ready and fit for purpose when things are more challenging.
"Notwithstanding our disappointment, Sinn Fein remains a very strong organisation."
The elections come following a disappointing performance for the party in the 2018 presidential election.
Party candidate Liadh Ni Riada attracted 93,987 votes, finishing in fourth place.
It was a significant drop from the 2011 presidential election in which the late Martin McGuinness scored 243,030 votes, finishing third.