He found it ironic that the person who helped him was the one who had deceived him years ago.

ironic 是形容词，来自名词 irony（讽刺）。irony 源自希腊语 eironeia，原指"假装无知"，后来演变为"反讽、讽刺"的意思。ironic 表示"具有讽刺意味的"或"出乎意料地相反的"。