It was Ireland's third consecutive win over the world champions and capped off a desperately disappointing Six Nations Championship for Andy Robinson's men.
After away defeats to Scotland and France, it was time for the Twickenham faithful to witness just how far their team have fallen since that memorable World Cup final night in Sydney in November 2003.
Andy Goode looked to have secured England an unlikely win when he struck an magnificent touchline penalty in the 74th minute to put the hosts three points ahead.
But Horgan's late try, scored despite the best efforts of Lewis Moody, gave Ireland a win they fully deserved.
Ronan O'Gara grabbed a personal haul of 13 points as the masses of Irish supporters left Twickenham delighted on St Patrick's weekend.
In stark contrast to last weekend in Paris, England started the game in superb style with Newcastle centre Jamie Noon flying over for a try in the second minute.
However, Ireland responded with a try from Horgan, although the winger appeared to have stepped into touch as he kicked ahead to score.
Steve Borthwick went over for the home side in the second half but Ireland hit straight back with a try for Denis Leamy after a horrible mix-up at an England line-out.
Leamy's try, converted by O'Gara, put the visitors three points ahead but a penalty from Goode after 68 minutes levelled the scores at 21-21.
And despite Goode kicking successfully again soon afterwards, Horgan struck decisively to break English hearts.