The conventional thinking is the Irish fall to #4, or the slimmer possibility is out of the playoffs altogether.
But not so fast, my friend. Ohio State's resume isn't all that sparkling, and with playing only 6 games, the committee may decide to keep the Irish at #3. While I don't put the odds of this as great, I do think the possibility exists.
Finally, remember the committee has always said they give the same weight to the first week as the last week, so the fact that when ND lost may not mean anything.