Legacy airlines sell a mix of refundable (even after a passenger was due to fly) and non-refundable tickets. The refundable tickets that were not used can sometimes still be refunded several months after the original planned departure date
LCCs tend to sell non-refundable tickets, some of which can be changed before, a flight or maybe a couple of hours after in case of accidental missed departure
Airlines report load factor no more than a couple of months after the month in question. For LCCs, they know a ticket not used more than 2 hours after a flight can never be reused or refunded, and the airline can book this in the accounts as revenue (minus possibly APD that would have gone to Govt anyway). Thus an LCC can choose to report either a 'bums on seats' load factor or a 'seats sold' load factor
Legacy airlines cannot book refundable tickets as revenue in the accounts until the refund period in the ticket fully expires. While they can estimate likely refunds, they cannot be 100% certain and accountancy rules thus prevent them reporting a 'tickets sold' load factor. A legacy carrier can only report a 'bums on seats' load factor
There are always more tickets sold than bums taking their desired seat... so LCCs choose to report the higher and more flattering 'tickets sold' load factor
Load factors from Ryanair and Aer Lingus are not exactly comparable - maybe add 5 percent to the 'bums on seat' load factor to get a 'tickets sold' load factor - but it is still a useful comparison tool