Ireland's goal came when Robbie Brady whipped in a corner which central defender John O'Shea headed powerfully goalwards. It appeared to strike a Dutch arm, but Long bundled the ball home.
The Irish lost their shape following a slew of second-half changes, allowing Dutch substitute De Jong to steal in and head home an equaliser with virtually their only decent attacking move of the game.
The Irish, who play Belarus in their final pre-Euro 2016 friendly in Cork on Tuesday, kick off their Group E campaign against Sweden in Paris on June 13 before meeting Belgium and Italy.
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With manager Martin O'Neill waiting until Tuesday to announce his final 23-man squad, he gave midfielders Stephen Quinn and Harry Arter as well as striker David McGoldrick a chance to stake their claims and all three rewarded him with energetic displays.
The Dutch may have looked disinterested throughout, but there was no denying the quality of Ireland's football in the first hour and the danger they posed from set-pieces.
The home side mixed up short passing and raking long balls to keep their opponents on their toes, and left back Brady created plenty of chances by whipping pinpoint free kicks and corners into the box at pace.
Despite the sloppy defending, O'Neill will not be too concerned about the concession of a late goal. His biggest worry now will be who to leave out of the squad for France.
(Editing by Ed Osmond)
