Probably from a primary verb eiro (to join); peace (literally or figuratively); by implication, prosperity -- one, peace, quietness, rest, + set at one again.

εἰρήνη (eirēnē) − 48 Occurrences

εἰρήνην (eirēnēn) − 25 Occurrences

εἰρήνης (eirēnēs) − 19 Occurrences