Being interested in words (surprise!), I looked up the first time eirene was used in the Apostolic Writings and found these words of the Master from Matthew 10.
"If the house is worthy, give it your blessing of peace. But if it is not worthy, take back your blessing of peace.
This is part of the instruction that Yeshua gives to his disciples as He sends them out. The conditional "if the house is worthy" should cause us to ask the question "what makes the house worthy?" The answer is found in the next verse:
"Whoever does not receive you, nor heed your words, as you go out of that house or that city, shake the dust off your feet. [emphasis added]
My favorite irenic passage is found in Romans 5.
Therefore, having been justified by faith, we have peace with God through our Lord Jesus Christ, through whom also we have obtained our introduction by faith into this grace in which we stand; and we exult in hope of the glory of God.
I'd like to close this short post with another irenic quote from Romans:
Now the God of peace be with you all. Amen.