As I understood the Middle English "un" ending means possessive Pronouns (ourun), third-person plural past participle (fallun, comun, wonnun) or having the quality of (wollun, lynun, goldun, stonun).
Why "un" ending are used in Middle English "eirun" eggs?
Wycliffe's Bible
Job.39:14
Studylight:
"which ostrige forsakith hise eirun in the erthe, in hap thou schalt make tho hoot in the dust"
King James Bible:
"Which leaveth her eggs in the earth, and warmeth them in dust…"