Ian Stapleton scored twice late on against his former club at Old Church Park to give Cobh Wanderers a second bite of the cherry.
Avondale went ahead on 27 minutes when a Dave Kiely cross was headed in by Gary Comerford over Shane Hallahan.
A second arrived two minutes later when Gary Collins' back-pass was intercepted by Simon Holland, who poked the ball into the net past Hallahan.
Wanderers pulled one back soon after when Conor Meade was fouled by Mark Horgan.
Referee David Keeler pointed to the spot and up stepped ex-Cork City striker Davin O'Neill to fire home to make it 2-1.
Avondale regained their two-goal cushion on 70 minutes, when a Karl Caulfield corner was headed powerfully to the net by Holland for his second goal.
However Cobh rallied, with ex-Avondale player Stapleton firing home a 20- yard free-kick to make it 3-2 on 73 minutes.
Then, with only five minutes remaining, Stapleton capitalised on a poor Horgan header to shoot past O'Leary from 14 yards and set up a replay for Wednesday.
Cobh Wanderers - Hallahan; Hastings (Bruton 77), Kavanagh, Fitzgerald, Collins; Geasley, Kinsella, Stapleton, Nolan (O'Flynn 77); Meade, O'Neill.
Avondale United - O'Leary; Cahil (Cairns 81), Horgan, Mulconry; Kiely, Caulfield, Comerford, Warren (Lougheed 70), Kelly; Long, Holland
Ref - D Keeler (Cork)