Etymology: Greek, eirenikos, from eirene, "peace" or "time of peace".
The similar-sounding ironic is also of Greek origin, but it derives from eironeia, "assumed ignorance", from eiron, "dissembler".
Most uses of the word seem to be from religious texts, but here's an example from the 1897 book The Literary History of the American Revolution, 1763 - 1783, by Moses Coit Tyler:
"...I have greatly mistaken the case if one practical consequence of this history...shall not be eirenic rather than polemic - namely, the promotion of a better understanding, of a deeper respect, of a kindlier mood on both sides of the ocean, among the descendants of these determined men who so bitterly differed in opinion, so fiercely fought, and in their anger so widely parted company a century and a quarter ago."
Big words. Then again, he was the first professor of American history.