Yeah I wonder that as well... I can't be the only person who, for the most part, travels on a weekend?
Maybe it's just lower yield passengers on Saturdays and they don't deem it worth operating the flight.
If Aer Lingus has less transatlantic flights on Saturdays I can understand why... Ryanair though, is mostly leisure, so I'd have thought there'd be demand
Without even looking yet, I'll put money that they both leave roughly, if not exactly the same time?