Seasoned observers of presidential races see it as a two-horse race between former Munster and Ulster chairmen Sean Walsh and Aogán Ó Fearghail.
However, ex-Leinster chair Sheamus Howlin is also understood to be picking up several votes.
Moyvane man Walsh is aiming to become the second Kerryman to take the office after Sean Kelly served three years between 2003 and ’06.
Should he become the 38th Uachtarán, Farrell will be the first Cavan man to do.
Howlin is aiming to follow in the footsteps of Michael Kehoe (1949-52) and Patrick Breen (1924-26).
This evening’s ballot is the first since 2008, when Christy Cooney defeated O’Neill to succeed Nickey Brennan.
O’Neill was elected uncontested in 2011. Howlin and former Ulster chairman, Donegal’s Tom Daly, had earlier pulled out of the race.
Meanwhile, security has been stepped up in Croke Park ahead of anticipated protests from local residents regarding this summer’s Garth Brooks concerts and the ongoing handball centre row.
Similar remonstrations have been made at recent Congresses with a number of protestors breaking into the 2011 congregation in the Mullingar Park Hotel.