It’s been awhile since Notre Dame played a great defense. You’d have to go all the way back to Michigan (5th according to S&P+) to find a time when the Irish played a top group on that side of the ball. Since then they’ve only played three more in the top-50: Northwestern (32nd), USC (40th), and Florida State (41st).
The Clemson defense they’re going to face three weeks from now is an entirely different beast.
They are the top ranked defense in S&P+ and the top defense in terms of yards per play given up as well.