Why we had two empty rows on busy Ryanair flight Boeing 737-800 and no one was allowed to seat there?
Were these emergency exit rows?

Seat pitch on low cost airlines is too small to allow an emergency evacuation within the required 90 seconds with regular seat pitch in the exit rows. Therefore, the seat pitch is larger there, which makes a world of a difference for tall people. Low cost airlines have decided to monetize those few more inches of leg room and charge extra for sitting there.

If no passenger on that flight was willing to pay the extra fee, those airlines which pride themselves on treating their passengers with the utmost contempt keep these emergency exit row seats free even if the plane is fully booked on all other seats.