Ireland's captain Andrew Balbirnie and vice-captain Paul Stirling put on 214 runs for the second wicket to guide the visitors to a record chase of 329 to win.
Balbirnie elected to field first for the first time in the series, and would have initially pleased as Jason Roy, Jonny Bairstow and James Vince were all dismissed before the score had reached 50. But Eoin Morgan and Tom Banton put together a 146-run stand to put the hosts back on top. Morgan was particularly belligerent as he hit his way to an 84-ball 106, his 14th ODI ton, including 15 fours and four sixes.
Banton moved to his maiden ODI fifty shortly before Morgan's dismissal, and he swiftly followed his captain back to the dressing room, becoming Gareth Delany's first wicket of the series. Ireland reclaimed even more momentum as Moeen Ali and Sam Billings were sent back cheaply by Curtis Campher and Craig Young respectively.
England mounted another fightback in the form of a 73-run partnership between Tom Curran and David Willey, who had scored 51 from 42 balls – including three sixes – by the time Campher had him caught at long-on. Curran shepherded the tail to get England past 300 before they were all out in the 50th over for 328.
Ireland set off steadily in pursuit of what would be their biggest-ever chase in ODIs, edging out their famous victory over the same opposition at the 2011 ICC Men's Cricket World Cup where they had chased down a target of 328 thanks to a 50-ball century from Kevin O'Brien. Stirling and Delany brought up the first 50-run opening stand of the series from either side before the latter was bowled behind his legs by David Willey to bring Balbirnie to the crease.
Stirling and Balbirnie quickly settled in and began to score freely off England's spinners. With Morgan off the field having picked up a groin strain while batting, vice-captain Moeen Ali was in charge, and he cycled through bowling options as England searched for a way to break the growing partnership.
Vince had claimed the Balbirnie's wicket in the second ODI, and he came close to dismissing the Ireland captain again as the batsman top-edged a swivel pull shot off his second ball. But the pair grew increasingly confident in their stroke-making as they carried Ireland towards their target.
England finally got the breakthrough as Stirling was run out for 142 – Ireland's highest individual score against England, and joint-fourth highest overall in ODIs. The partnership was also Ireland's highest against England, and second-highest in all ODIs. Balbirnie fell to Rashid's leg-spin a few overs later, holing out to long-off for 113, to leave youngster Harry Tector and the experienced head of Kevin O'Brien needing to score 50 runs from 33 balls.
The pair found boundaries at crucial moments and, helped by a couple of full-toss no-balls, they settled it with a ball to spare to give Ireland their first points of the ICC Men's Cricket World Cup Super League.
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