Trailing by a point with a minute of normal time left, the east Cork side finished strongly, Paudie O’Sullivan equalising and then, in injury-time, Cotter fired over after a frantic passage of play around the Cill na Martra 45.
A tally of 1-6 from O’Sullivan was crucial, the attacker effective at corner-forward in the first half before being relocated to centre-forward for the second period.
His brother Diarmuid had manned that post early on, effectively at times, but caused a lot of damage early in the second half, necessitating the switching of Cork star Nollaig Ó Laoire to mark him.
While that move diminished O’Sullivan’s threat, it proved to be a double-edged sword as Ó Laoire was unable to provide the same influence that he had done earlier at centre-back, his surging runs a key factor in Cill na Martra attacks.
In addition, in Caoimhín Ó hUidhir, the Gaeltacht side had a ready source of scores in the first half, the corner-forward scoring five points as they led by 0-10 to 1-5 at half time.
They were forced to battle hard to get to such a position though, scoring the last six scores of the half. Prior to that, Cloyne had been threatening, seizing the lead when Paudie O’Sullivan blocked down a Caoimhín Ó hÉalaithe clearance in the 10th minute before finishing coolly to the net.
That made it 1-2 to 0-3 and, by the 19th minute, they were four ahead, 1-5 to 0-4.
Cloyne kicked three bad wides before half time, and they looked to be up against it at the turnaround.
Caoimhín Ó Súilleabháin pushed the Muskerry club further clear three minutes after the turnaround, but slowly the tide began to turn Cloyne’s way as they got on top in the air, Diarmuid O’Sullivan pointed and then went close to a goal before two frees from his brother had the scores level by the 40th minute.
Now it was Cill na Martra’s turn to waste good scoring chances, but while Ó hUidhir was not as potent as in the first half, his frees were keeping them above water, and when Máirtín Ó Conchúir pointed on 55 minutes they led by 0-14 to 1-10.
It was their last score as Cloyne turned it around — although Maidhcí Ó Duinnín had a late equalising chance that he punched wide.
Cill na Martra now face Killavullen in their last chance to reignite their championship hopes, while Cloyne advance to meet Ballydesmond in the quarter-finals.
Scorers for Cloyne: P O’Sullivan 1-6 (0-3f); J Nyhan 0-2; J Cotter, D O’Sullivan, P O’Sullivan, C O’Sullivan.
Scorers for Cill na Martra: C Ó hUidhir 0-7 (6f); C Ó Súilleabháin, M Ó Conchúir 0-2 each; D Ó Conaill, S Ó Ceallachair, M Ó Duinnín 0-1 each.
CLOYNE: C Smith; J Cotter, Domhnall O’Sullivan, D Jermyn; G Fleming, E O’Sullivan, B McCarthy; K Cronin, M Lynch; C O’Lomasney, D O’Sullivan, P O’Sullivan; P O’Sullivan, C O’Sullivan, J Nyhan.
Subs: K O’Lomasney for C O’Sullivan (24), M Walsh for Lynch (38).
CILL NA MARTRA: E Ó Duinnín; C Ó hÉalaithe, C Ó Laoire, G Ó Mocháin; S Ó Loingsigh, N Ó Laoire, C MacSuibhne; S Ó Ceilleachair, T Ó Duinnín; M Ó Duinnín, M Ó Goillidhe, D Ó Conaill; C Ó Súilleabháin, M Ó Conchúir, C Ó hUidhir.
Subs: D Ó Mocháin for T Ó Duinnín (51), C Ó Loingsigh for Ó hÉalaithe (58), A Ó Mocháin for Ó Goillidhe (60).
Referee: Martin Collins (Clonakilty).