Don't worry, when you arrived first at that island Biggs or Wedge should have mentioned that they've set up a Garlond Ironworks airship route between that point and Ishgard in case you needed to return to the city in a hurry.

Hence, at the airship dock in Ishgard's Pillars district there should be a member of the Garlond Ironworks standing in front of the departure gate who if you speak to them will fly you automatically back to that island in the Sea of Clouds. Additionally if you need to return to Ishgard again you can do so without teleporting/returning by taking another airship in reverse by clicking on the 'Ironworks Standard' sign.

Either way, don't despair! You're not stuck so please don't worry! Just go back to the airship dock at Ishgard and you should be able to get back to where you need to go.