Donegal Celtic 1, Ballymena United 3 (after extra-time)
The Sky Blues were facing elimination at Suffolk Road until Cushley - the club’s top scorer for each of the past two seasons - struck to send the tie into extra-time.
United manager Glenn Ferguson had reshuffled his starting line-up including returns to the starting line-up for Kyle McVey and Alan Davidson following injuries.
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Hide AdBut United were dealt a blow in the second half when Mark Surgenor was shown a straight red card for a late tackle on Emmett Templeton.
And it got worse for United on 72 minutes when Ryan Dunlop headed Celtic ahead from close range.
Ballymena, the 2012 Shield winners, were seemingly heading out of the competition until Cushley drilled home a volley through a crowded penalty area on 88 minutes.
In extra-time, Cushley latched onto a Darren Boyce through ball to put United ahead.
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Hide AdAnd the same player completed his hat-trick from the penalty spot to ensure Ballymena’s place in the quarter-finals.
Ballymena United : Nelson, Ruddy, Stewart, McVey, Kane, Surgenor, Davidson, Gawley, Teggart, Boyce, Cushley. Subs: Thompson, Tipton, Harris, McBride, McCullagh.