I believe the answer to the OP’s question is that Ryanair do not carry cargo in any form. However they have made an exception for human remains, in coffins, on this one route. Likely because of political pressure or because they have made a very good deal with a specialist shipper.
Said cargo moves at a multiple of the most expensive rate per kg, and a coffin and it’s contents are heavy.
Transporting ashes, otoh, is much less complicated and costs nothing extra. The airline doesn’t care, as long as you can get through security.