Their skin is very dry but their insides contain compressed excesses of liquid. Because of this, their body chemistry is volatile and concentrations of adrenaline can allow them superior speed and strength for short periods of time.
Irkens are sensitive and irritable when exposed to many Earth-based resources, but few (if any) materials can cause lasting injuries.
It is estimated that they are reptilian based off of the Mysterious Mysteries host describing Zim as having scales. However, Irkens do not seem to mate in modern society, being genetically manufactured (see Gattaca). Despite being presumably reptilian, Irkens have very humanistic proportions as well as mammal-like tendencies, behaviors, and movements.
Lastly, Irkens are a form of cyborg, their PAKs being cybernetic technology fused into their neural system, allowing them the use of four extra limbs (each containing a laser), and a communication device.
PSYCHOLOGY AND SOCIETY:
Irkens are very similar to humans in having megalomaniac and dominative tendencies. Their culture is similar to Asian culture, the civilians being hardworking and loyal to the Almighty Tallest.