jonathang: Most airlines would not phone and advice passengers of a time change, not just Ryanair.
In my experience, this is not true. Most airlines will try to contact you in the event of a major schedule change,
if they have your contact details. The last time I was contacted by an airline (BA), it was to tell me that my flight was going to leave
15 minutes earlier than previously notified. Many friends who fly on US domestic flights, where schedule changes are much more frequent, have similar experiences.
Risk you take when you book two single Sector flights. The carrier does not know your plans. The responsibility is to get you from A to B.
This is also just not true if you have booked both sectors on the same PNR. FR would have known perfectly well that it was intended to be a day trip. But it sounds like they did the right thing by allowing the trip to be cancelled.
FR did the same for me on the one and only time I have booked with them - ironically a day trip taken just to see what the "Ryanair experience" was all about. The outbound flight was delayed and delayed and delayed. The aircraft for the return flight was not - I could see that it was boarding and ready to depart on time. I called FR who could immediately see that if I waited for my outbound flight to finally depart I would definitely miss the return flight, so they canx the res at that stage and gave me a full refund. That was also the right thing, and it confirmed to me that I shouldn't clamour to fly FR again.