Do I need a VR headset to use IrisGL?
No, you can compile and implement the engine in your own application and use it as normal to create worlds and add special effects and share these. A HMD is recommended of optimal experience however but is not needed.
IrislGL currently supports Oculus Rift in the developer version, but support is planned for other VR platforms including the Vive and Google Daydream. The goal is the be device independent.