The Illyrian starship was a class of starship utilized by the Illyrians during the mid-22nd century. This class of vessel was equipped with a minimal armament of particle weapons and was tactically inferior to an NX-class starship. This class was also notable for having its primary warp coil, which was tied into the ship's injector system, protected by a force field.
In 2154, the badly-damaged Starfleet vessel Enterprise NX-01 encountered a damaged Illyrian starship in the Delphic Expanse. The Illyrian starship was damaged by the spatial distortions of the Expanse while studing a red giant. The distortions caused damage to the ship's engines and life support.
Captain Jonathan Archer asked the Illyrian ship's captain for his warp coil in exchange for food and supplies. The captain refused, as the loss of their warp coil would leave them with a three year journey to their homeworld – a journey for which his ship was not equipped. As Enterprise required the warp coil in order to continue their mission to find and destroy the Xindi superweapon before it destroyed Earth, Archer was forced to take an away team aboard the Illyrian starship and steal the coil.
Aboard the Illyrian ship, the Enterprise crewmembers encountered a force field surrounding the warp coil. It was only after T'Pol had Enterprise's phase cannons disable the ship's power junction that was supplying power to the force field, that the force field was eliminated, allowing the away team to remove the coil.
Before leaving the Illyrian ship, Archer had Enterprise transport food, supplies, and three containers of trellium to the Illyrians' cargo hold to assist them in their three-year journey back to their homeworld. (ENT: "Damage")