Cllr Alan Coleman will relinquish the chain of office tomorrow and, after a short family holiday, will announce his future political plans.
According to sources, Mr Coleman, a lifelong member of Fianna Fáil and a county councillor for 24 years, has become extremely disillusioned with the party.
Last weekend, he was passed over in the Cork South West constituency as a Dáil candidate.
He withdrew his name before delegates voted at a constituency meeting in Bantry.
Mr Coleman had also been passed over in 2011 as a likely candidate after long-serving TD and minister Joe Walsh did not seek re-election. Mr Walsh died in November 2014.
Fianna Fáil, for the first time in its history in Cork South West, did not win a seat at that election.
Mr Coleman, from Riverstick on the eastern side of the sprawling constituency, was first elected to the council in 1991.
He had reportedly been fostering relations with independent-minded people in the constituency in recent weeks.
If, as expected, he takes an independent route, it is believed he could do significant damage to his former party.
Bandon-based Margaret Murphy-O’Mahony, a first-time county councillor, secured the nomination at the selection convention.
In a constituency where, prior to the economic crisis Fianna Fáil had a secure 35%-40% of the the vote, it is expected that Ms Murphy-O’Mahony will be the only Fianna Fáil candidate to seek a Dáil seat.
On that basis, she has an exceptional chance of being elected, representing a rapid political rise for the former Bandon town councillor.
Currently, Fine Gael and Labour control all three seats in the constituency.
If Mr Coleman enters the fray, however, it could impact on the party vote, particularly in the heavily populated eastern side of the constituency.
Fianna Fáil senator Denis O’Donovan, based in Bantry on the western side of the constituency, did not seek the nomination.
Sources say Mr Coleman’s year as mayor in Cork county would enhance his chances of snatching many Fianna Fáil votes throughout the constituency.
Neither he nor Ms Murphy-O’Mahony would have a strong profile in the Skibbereen, Bantry, and peninsula areas of the region.
Mr Coleman’s expected resignation will come as a major blow to colleagues.
He had recently signalled his disappointment to a number of high-profile members of the party.
It is believed that unless the Fianna Fáil hierarchy offers him a significant carrot, his departure seems inevitable.
It was believed locally that after Mr O’Donovan had indicated he would not seek a nomination, Mr Coleman was the heir apparent.
However, Mr O’Donovan backed Ms Murphy-O’Mahony, leaving the writing on the wall and leading to Mr Coleman withdrawing his name from the selection convention.
“I know when I’m not wanted,” Mr Coleman said in recent weeks.
“I wasn’t happy with the whole process. It was very clear to me from early on that headquarters was not happy about me seeking the nomination.
“The real pivotal issue is when the polls were done, my name wasn’t included in any of them — internal polls by Fianna Fáil.
“I was very disappointed with that.”