In Generation IV Dragonite can learn Iron Head in Pokémon Platinum, Heart Gold and Soul Silver through the Move Tutor. The Move Tutor who will teach Dragonite Iron Head is located in the Survival Area in Platinum and requires 6 red shards and 2 yellow shards to teach it the move. The Move Tutor is located in the Frontier Access in Heart Gold and Soul Silver and requires 40BP to teach it the move.
In Generation V Dragonite can learn Iron Head in Pokémon Black and White 2 through the Move Tutor. The Move Tutor who will teach Dragonite Iron Head is located in Driftveil City and requires 4 red shards to teach it the move.
In Generation VI you cannot teach Dragonite Iron Head in the game. So the only way to get a Dragonite in Pokémon X and Y with Iron Head is if you teach Dragonite Iron Head on one of the Gen IV or V games I've mentioned above and transfer it over using Pokémon Bank. Note if Dragonite was on Gen IV you would have to transfer it to Gen V before transferring it to Gen VI.
Hope I helped :)