What are the difference between iritis and allergic red eye?

Iritis: Iritis is a more serious condition it's related to many underlying problems. It can look like an allergic reaction with: varying degrees of redness of the eye, often with significant pain, sensitivity to light, tearing, and blurred vision. Allergy can happen anywhere in the body and the symptoms are mainly due to release of histamine.

Not the same: Iritis is an inflammation inside the eye--usually the front chamber which can make the eye red as well. Allergic red eye is inflammatory change induced by allergy that only effect the outside membrane of the eye. The treatments are different.