Nearly two-thirds of organisations that advertised during the referendum campaign were unregulated.
Analysis by the Transparent Referendum Initiative found that online advertising has been used extensively in the past few months even after Google and Facebook tried to clamp down on it.
Since February 14 the Initiative found that there were nearly 1,300 Facebook ads shown to 600 Irish people, with 58% associated with repeal and 42% with retain.
It also says that nearly one in 10 of the unregulated advertisers were based outside of Ireland, while a similar number were untraceable.
Co-founder of the initiative Craig Dwyer said: "What we have found is that our democracy is up for sale to the highest bidder.
"In many cases that bidder remains unknown and can get around the attempts by the likes of Facebook and Google to self-regulate."
Elsewhere, questions are being asked about the influence foreign financial interests have had on the abortion campaign.
The Irish Council of Civil Liberties says removing Maser’s 'Repeal the 8th' mural from the Project Arts Center in Dublin had a ‘chilling effect’.
Executive Director Liam Herrick says its about creating a balance.
He said: "Make sure the financial influence doesn't corrupt our politics which is absolutely essential but at the same time, allow different voices to have a voice in our society."
- Digital Desk