Casey, 35 birdied three holes in succession in a four under par first round on a rain-softened Carton House course.
he England golfer reached a high of number three in the world four years ago but teed up this week in Dublin having slipped to 169th and not having won since capturing the inaugural 2009 Volvo Champions event in Bahrain.
Casey, who also had been sidelined with a broken shoulder at the start of 2012, had to find his way into the US Open the hard way, qualifying weeks earlier at the Walton Heath course in Surrey.
"It was the first time ever I had to qualify for a US Open made me realise how precious they are playing in every one of the Majors," he said.
"And after seeing good friend Justin Rose win the US Open I feel as though I can play that kind of golf.
"Justin and myself grew-up together playing amateur golf, so I've known him an awful long time.
"But was I ready before seeing Justin win? I don't know but I know I am ready now. Certainly 10 years ago when I was pretty green I was not prepared to handle everything that goes with winning a Major.
"But that I feel I am fully, and I just need the golf to be there."
And Casey, who is a member of the Tournament Players Committee, and the body that advises the Tour on day-to-day issues affecting the players, was quizzed following Graeme McDowell's pre-event comments.
McDowell indicated more needs to be done to attract Europe's best players who are competing full-time on the PGA Tour back to play in Europe.
However Casey, who lost his PGA Tour card at the end of 2011, sees the issue more a tax-based concern.
"One of the main reasons the guys playing in the States don't come back to Europe more is the tax reasons and it's something that we can work on ourselves, and something the Tour and the R and A, if you are talking about the Open Championship, should try to address," said Casey.
"Being taxed on percentage of global income is a bit harsh and if that is affecting our game, and I am not saying standing here that it is, but it is something certainly the Tour needs to pursue.
"I'm on the Players Committee and we have huge opportunities but it is down to George (O'Grady, CEO European Tour) and his management team as well as the Tour's Board of Directors to sort out.
"We need direction as there is only so much the Players Committee can do.
"Then again the economic climate in Europe has never been more dreary and the opportunities I hear about are not are quite often not in Europe and away from Europe."