Earlier in the day, a six-try point victory over Italy had moved the Irish onto 10 points at the top of the table.
To reach the semi-finals they required a South African win this evening - but without a four-try bonus point - and they also needed England to lose without getting a bonus point.
In the end, Shaun Adendorff's 68th-minute try was enough to seal the Baby 'Boks' place in the last-four. Ireland finished second in the pool above England and were seeded sixth for the play-offs.
The English were seeded seventh so the teams are set for their third meeting of the season, with Mike Ruddock's charges all out for revenge.
Ireland lost the Grand Slam decider to England at Adams Park in March, going down 20-9. They were on course to win last Friday's pool clash in Stellenbosch, but England cancelled out a 15-3 half-time deficit as they came from behind for a 20-15 win.
But confidence is high in the Irish camp as they aim for a fifth place finish overall, which would represent an improvement of three positions from last year.
2012 IRB JUNIOR WORLD CHAMPIONSHIP -
POOL B RESULTS:
Monday, June 4: England 64 Italy 5, University of Western Cape Stadium, Cape Town
Monday, June 4: South Africa 19 Ireland 23, Danie Craven Stadium, Stellenbosch
Friday, June 8: South Africa 52 Italy 3, University of Western Cape Stadium, Cape Town
Friday, June 8: England 20 Ireland 15, Danie Craven Stadium, Stellenbosch
Tuesday, June 12: South Africa 28 England 15, Cape Town Stadium, Cape Town
5TH-8TH PLACE PLAY-OFFS -
Sunday, June 17: France v Australia, Newlands Stadium, Cape Town, 2.45pm local time
Sunday, June 17: Ireland v England, University of Western Cape Stadium, Cape Town, 5pm local time/4pm Irish time
7TH-8TH PLACE PLAY-OFF -
Friday, June 22: Losers of France/Australia v Losers of Ireland/England, University of Western Cape Stadium, Cape Town, 4.30pm local time/3.30pm Irish time
5TH-6TH PLACE PLAY-OFF -
Friday, June 22: Winners of France/Australia v Winners of Ireland/England, Newlands Stadium, Cape Town, 2.15pm local time/1.15pm Irish time
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