Sinn Fein's Dublin office has paid for Facebook advertisements promoting a party event in Belfast, analysis of social media reveals.
Sinn Fein's headquarters in the Republic also paid for advertising on Facebook for politicians running in Westminster elections.
The payments are to be reported to the Standards in Public Office Commission (Sipo) by Fine Gael.
It comes after concerns were raised about a €4m political donation Sinn Fein in Northern Ireland received from reclusive millionaire William Hampton last year.
Strict laws on political funding in the Republic prohibit parties from receiving a donation of more than €2,500.
Fine Gael has insisted Sinn Fein should return Mr Hampton's donation back to his estate due to concerns the money will be use to fund activities in the South.
When Mr Hampton's donation was examined by Sipo, Ms McDonald said her party operated separately on "a six-and 26-country basis" on political funding.
However, Sinn Fein's advertising spend on Facebook shows the party's office in Dublin has paid for content promoting activities in Northern Ireland.
In October, Sinn Fein's Dublin office paid between €200 and €299 for Facebook ads to promote an online event involving Deputy First Minister Michelle O'Neill.
In December 2019, Sinn Fein in Dublin paid for advertising promoting Michelle Gildernew and Elisha McCallion's campaigns to be elected as MPs. Ms McCallion was recently forced to resign from Sinn Fein.
Fine Gael TD for Carlow-Kilkenny John Paul Phelan said the spend on Facebook ads is "positive proof" Sinn Fein uses money and resources "across both jurisdictions on this island".
Sinn Fein said there is "no issue" with their finances and accused Fine Gael of "spin".
"The fact that they have nothing better to be doing at a time when they should be dealing with the Covid-19 pandemic, the housing crisis and the fact that tens of thousands of workers are out of work is pretty telling," a spokesperson said.