However an airline may not add more seats than the aircraft is certified for (i.e by government regulators). In the case of the B737-800 operated by Cryanair the FAA has certified this aircraft for a maximum of 159 passenger seats. The UK and other EU authorities have accepted the FAA's certification meaning that it is also applicable in the EU.
As Cryanair's fleet of B737-800 aircraft is already configured to the maximum 159 seats they cannot add any more seats (or standing-up seat variants). Of course they could take out as many toilets as they want (there are no rules on minimum numbers), but they couldn't add seats.
159?
Add 30 more seats and you've got it
