Ferrie erased a seven-shot deficit with a 2-under 70 in the final round to finish at 3-under-par 285.
Colin Montgomerie posted a 3-under 69 on Sunday at The K Club to share second place with Graeme Storm, who carded a 1-over 73 in the final round. The duo came in at minus-1.
Thomas Bjorn held a four-shot lead at the start of Sunday's final round, but struggled and found himself down one shot to Ferrie on the par-4 17th tee. Bjorn hit three balls left into the water and walked off with an 11.
Bjorn bogeyed the 18th in a driving rain that almost left the green unplayable. He shot a 14-over-par 86 in the final round and tied for 33rd at plus-6.
Darren Clarke and Peter Hanson both shot rounds of 1-over 73 and tied for fourth place at even-par 288.
Andrew Coltart was one back on the 17th green, but three-putted from just inside 2 feet. He ended up with a 4-over 75, but shared sixth with Brian Davis (71), Trevor Immelman (73), Jose Manuel Lara (73), Damien McGrane (74), Gary Murphy (72) and Jamie Spence (73) at plus-1.
Retief Goosen, the defending champion and two-time U.S. Open champion, managed an even-par 72 on Sunday and tied for 13th at 2-over-par 290.
Michael Campbell, who won his first major title a few weeks back at the U.S. Open, posted a 3-over 75 and shared 25th at plus-4.