Cusack took two wickets in three balls in the final over as Ireland knocked Zimbabwe out of the World Cup with a thrilling five-run win in their Pool B match at Bellerive Oval in Hobart on Saturday.
The African side, chasing Ireland's total of 331 for eight, were bowled out for 326 with three balls remaining to become the first test-playing nation to be ruled out of contention for a place in the quarter-finals.
With Zimbabwe needing just seven runs in the last over, Cusack (4-32), recalled to the side in place of Max Sorensen, took a wicket on his first and third balls to spark wild celebrations among his team mates and their supporters.
This was Ireland's third win in four matches and put them level on six points with South Africa and Pakistan with a game in hand. West Indies have also played five matches and are on four points.
Earlier, left-hander Ed Joyce lived a charmed life to hit his first World Cup century and was well supported by Andy Balbirnie, who was run out on 97, as Ireland posted their highest one-day international score after being put in to bat.
Brendan Taylor (121), leading Zimbabwe in place of injured captain Elton Chigumbura, hit his seventh ODI hundred, matching Alistair Campbell for the highest number of 50-over centuries for Zimbabwe.
Taylor and Sean Williams (96) combined in a 159-run stand for the fifth wicket to put Zimbabwe back in the hunt after they were reduced to 74-4.
Williams continued to plunder runs after Taylor, who hit 11 fours and four sixes in his 102-ball knock, got out to Cusack.
The left-handed Williams was out in controversial fashion while going for a six. John Mooney held on to his catch at square leg off Kevin O'Brien, but it was unclear from replays whether the fielder's foot was in contact with the rope.
The dismissal of Williams almost ended Zimbabwe's hopes of a win before Tawanda Mupariwa (18) hit O'Brien for two fours and a six in three balls in the penultimate over to bring the victory target within reach.
Joyce (112), who hit nine fours and three sixes, edged his first delivery off Tinashe Panyangara but the ball fell just short of second slip. Then he was dropped on 34 and again after completing his third one-day international hundred.
The 36-year-old left-hander and captain William Porterfield (29) added 63 for the second wicket as Ireland made a watchful start after being put in to bat on a sluggish pitch in Hobart, hosting its first match in the tournament.
The momentum picked up during an Irish third-wicket record 138-run stand between Joyce and Balbirnie before the former was finally caught by Craig Ervine, who dropped him in the previous over, off Tendai Chatara.
Balbirnie 24, looked set for his maiden ODI hundred only to run himself out on the first ball of the final over trying to go for a second run to get back on strike.
Seamer Chatara and left-arm spinner Williams picked up three wickets each for Zimbabwe.