Ms McGuinness has 135,698 votes after she got an astonishing 72% of Jim Higgins' available transfers - 28,009 out of 38,429 transferrables - to put her ahead of Sinn Féin's Matt Carthy on 126,492 votes.
Independent Luke Ming Flanagan was declared elected following the second count yesterday, so after McGuinness' election, there are now only two seats left.
Pat the Cope Gallagher of Fianna Fáil's only hope of victory is a similar inter-party transfer, he will need well over 50% of Thomas Byrne's votes to pip Harkin.
Standings after fifth count:
Mairead McGuinness (FG) 135,698,
Matt Carthy (SF) 126,492
Marian Harkin (Ind) 95,577
Pat "The Cope" Gallagher (FF) 67,606
Thomas Byrne (FF) 63,392