My best guess is The Fairs Act of Ireland from 1204 CE. Page 12 of this pdf [warning] of the Irish Statute Book, under the Schedule titled "Statutes Retained" indicates that it's the oldest law still in effect in Ireland. I assumed maybe Iceland or Japan might have something older, but I don't know of anything and couldn't find anything on any my databases.

I'm an attorney rather than an historian, but most of my journal work related to really old (most copyright) laws in Europe.