I'd argue that the 9/11 attacks were the major turning point. It's not a coincidence that the Provos agreed to surrender their arms a little over a month after that event. The Americans who had traditionally kicked money to the IRA were suddenly less sympathetic to the argument that "one man's terrorist is another man's freedom fighter."
The McCartney murder brought widespread attention (at least on this side of the Pond) to how much the IRA had changed, in public perception if not reality. The Good Friday accords were twelve years ago. There is a whole generation of thugs who joined the IRA since then without any nationalist ambitions. They're just thugs who want to be thugs, and their version of the IRA is nothing more or less than a red-headed Mafia.
I don't use the term "Mafia" carelessly. The Sicilian Mafia formed to resist French occupation. When the French were gone, it became a criminal gang oppressing the people it had been formed to liberate. The IRA appears to be following the same path.
I'd argue that most American support for the IRA dried up after 9/11. The McCartney sisters -- make no mistake, I salute their courage and eloquence, and admire them greatly -- were just the final nail in the coffin. Irish-Americans came to realize that the IRA of today is not made up of the people their grandmothers sang folk songs about.
Actually, the IRA are arguably stronger than they ever were - they are now engaged in politico-criminal conspiracy to overthrow Ireland's democracy, and replace it with a facsist-Marxist dictatorship.
And all Bertie Ahern does is appease them!