The Sinn Féin leader has accused the government of engaging in "rhetoric", over suggestions it can not fix an issue preventing women who return from maternity leave, accessing the wage subsidy scheme.
The scheme covers up to 85% percent of a person's wages, allowing companies to keep them on payroll during the current crisis.
Mary Lou McDonald believes the issue can be fixed and says it needs to be sorted very quickly.
She says: "For women in this situation who have been out on their maternity leave who now have a newborn child at home, and all of the stresses and strains that every family across the country is now under because of the public health emergency.
"Really to be excluded in this way is just for many people the last straw."
Sinn Féin leader Mary Lou McDonald has called for the temporary wage subsidy scheme to be fixed, so women returning from maternity leave are eligible.
The scheme involves the Government subsidising up to 85% of wages for companies hit hard by the crisis to keep staff on payroll.
Last month an irregularity was found that prevented companies from accessing it for women returning from maternity leave.
Mary Lou McDonald says this cannot be allowed to continue and needs to be fixed.
"It is unfair. It is arguably discriminatory to put these new mothers in this position," she says.
"The Government seems to be suggesting that they can't actually fix this anomaly. But I don't believe that is the case.
"I have therefore written to all of the party leaders to urge that we sort this out very very quickly."
Yesterday Sinn Féin called for the pandemic unemployment payment to be extended until the end of the year.
The benefit of €350 a week is available to those whose income has been affected by Covid-19.
It is in place until June 8, and there has been no decision yet on its extension.
Sinn Fein's Finance spokesperson Pearse Doherty said it makes economic sense.
"What's very clear is the cost of actually cutting the supports will be worse for the economy because we have a situation where there is a dampening down of consumer confidence," he said.