You must distribute the iPrint app to devices in your organization via MobileIron if this is the first time your organization is using MobileIron with iPrint, or any time a new device enters the organization.
Some users might have independently download the iPrint app from the app store before their device is managed by MobileIron. Even in this scenario, you must push the app to their device via MobileIron. (These devices will lose any cached or downloaded files within the iPrint app after their device becomes managed and the iPrint app is pushed to their device.)