The ballot in Ballybay Clones will have to be reheld at a later date following the death of Fine Gael candidate Owen Bannigan.
The Department of Environment has said that the election to Monaghan County Council in the Ballybay Clones district is now cancelled, and all ballots cast are null and void.
A separate election will be held in the coming weeks.
The polls remain open in the Ballybay-Clones electoral area until 10pm to facilitate voting in the European elections - and votes already cast in that election are still valid.
He was running for re-election in the Ballybay-Clones electoral area, but died suddenly of a suspected heart attack earlier today.
Fine Gael's General Secretary has paid tribute to the Councillor, who died while canvassing.
Tom Curran said Councillor Bannigan had showed great dedication in serving the people of his area.
The 54-year-old was a farmer by profession and held a seat in the Ballybay-Clones area since 1999.
The ballots in the separate electoral areas of Monaghan and Carrickmacross-Castleblayney will continue as planned, as well as voting for the European Elections in the Ballybay-Clones electoral area.
Polling stations close at 10pm.