The fire at Peak Environmental Recycling Plant on the Clooney Road broke out before midday on Tuesday.
Fire crews found a "fully developed fire" in several buildings on the site, and a 200 metre exclusion zone has been set up.
"The buildings contain large quantities of plastic and cardboard and diesel are also involved in the incident," said a Fire Service spokeswoman, who added it is too early to determine what caused the blaze.
Although the building was also thought to have contained a number of gas cylinders, the Fire Service later discovered there were no acetylene cylinders at the site.
Five fire engines and four specialist appliances are at the scene, and the Fire Service expects to remain there for several hours.
Brenda Stevenson, a local SLP councillor said bringing the fire under control was "a very very dangerous operation for anybody involved".
"We have to pay tribute to all those firefighters who went down and risked life and limb to try and bring this very serious fire under control," she said.
Residents have been advised to keep their windows closed because of smoke pollution in the Clooney Road, Cloghole Road, Kilnappy Road and Strathfoyle areas.