One of the reasons for flags of convenience is that it allows the ship's owners to bypass rules on who they employ. EU registered ships need to employ a certain percentage of EU resident crew and presumably pay them more than non-EU crew. Certainly on DFDS ships the crew appear to be mostly from the Far East.
I imagine this business model doesn't really work for airlines such as Ryanair so there is no need to register abroad.
I was a us merchant seaman in a former life, and worked on a chemical tanker (600' floating mothball) that only serviced the us eastern coast and us gulf of mx. We would fill up with du pont & monsanto nastiness in ports around houston & louisiana and discharge in and around nyc - lovely spots like bayonne, nj & chester, pa. My job was crew messman in the stewards dept - got to serve food to non-officer crew and was chief bottle washer.
Sorry, my point is that since the vessel only worked us coastal/inland waterways it was required to be register in the us (philadelphia) and employ only us reg seaman.
I graduated from the stewards dept to ordinary seaman after that ship and got to work on deck; had enough seatime w/union to move to higher group level for better jobs. Next ship was another tanker but took subsidized shipments to impoverished areas - we took wheat to egypt on one trip. This was in the mid 1980s and shortly afterwards us shipping cratered moving to foreign ships & crews. Sadly, I couldn't find any shipping work and had to find a real job.