Species of dragon native to eastern Europe
habitat: mountains (presumed) – possibly Northern Carpathians, Transcarpathian Region that borders Romania and Hungary
appearance: metallic grey, long talons
size: largest breed of dragon, up to six tons
eyes: deep red
The Black Sea borders Ukraine to the south, and FB particularly mentions that Ironbellies have been known to hunt in that area.
The Transcarpathian Region of the southwestern Ukraine seems a likely candidate for the primary habitat of the Ironbelly for various reasons. It borders Hungary and Romania, both of which support dragon species of their own, so the region where the three countries touch is worthy of consideration as a dragon habitat for that reason alone.
The Northern Carpathian Mountains running through the Transcarpathian Region of the Ukraine continue into the Southern Carpathians, also known as the Transylvanian Alps. In addition to being a mountainous area, and thus a common sort of dragon habitat, the Carpathians are sparsely inhabited by Muggles, which would help explain how the wizarding world has managed to hush up the existence of the Ironbelly, given that much of the rest of the Ukraine is very thoroughly in use by Muggles as farmland.