Seven years after three-putting the 18th green at Carton House to lose out by a single shot to Thomas Bjorn, Casey needed just one putt from 45ft on the same par-five to record an eagle three and seal a 12th European Tour victory.
A closing 67 - four shots better than anyone in the last 12 groups in strong winds and a brief torrential downpour - gave the 35-year-old a total of 14 under par, three clear of playing partner Robert Rock (71) and overnight leader Joost Luiten, who had been looking for his second win in three events but could only manage a 74.
"Psychologically this is huge," said Casey whose last European Tour win came in Bahrain in January 2011. "It feels like a first win again. I have struggled with the confidence and this is a huge relief knowing I am moving in the right direction.
"I have great golf in me for 10 years plus. This is a massive confidence boost."
Casey will now turn his attentions to trying to win the Scottish Open to qualify for the Open Championship at Muirfield, with Rose's recent US Open triumph a massive inspiration.
"That really lit the fire," added Casey, who witnessed Rose's win first-hand after coming through qualifying with rounds of 74 and 64 at Walton Heath. "Justin's victory was phenomenal and I was so proud of what he did and how he handled himself.
"It's well documented how he struggled at the beginning of his career and I have seen the work he has put in to become one of the best players in the world. I have been there before and I desperately want to be in control of my game and winning championships again.
"I was ready back in 2009 when I was playing some phenomenal golf and I'm really ready now for everything that happens after a major victory. That's the goal."
Casey was ranked third in the world in 2009 but began the week 169th after form and fitness problems, most notably after suffering a broken collarbone while snowboarding in late 2011.
He also went through a painful divorce around the same time and admitted he feared that he would never rediscover the form which brought him three consecutive Ryder Cup appearances - the first two of them record wins at Oakland Hills and the K Club - from 2004.
"It's difficult to pinpoint a moment but there was a period where I really struggled with getting the shoulder back where I wanted it and I probably came back too early because the swing changed trying to protect the shoulder," added Casey, whose win takes him back into the world's top 100.