Now customers are urging the budget airline to list all of the flights that they intend to cancel over the next six weeks so they have enough time to make alternative plans.
The Irish no-frills airline said Friday it would scrap up to about 2,000 flights over six weeks as it struggles with pilot availability and is hoping to restore a high level of punctuality.
The airline has said it is offering full refunds or alternative flights to those affected, although many passengers have complained that the alternative flights on offer are days later than they were originally due to fly.
Hannah Maundrell, editor in chief of price comparison site Money.co.uk, says: "It's so disappointing for Ryanair passengers whose flights have been cancelled especially at such late notice".
It says passengers are entitled to a full refund, or an alternative flight.
"Norwegian welcomes any initiatives that offer passengers smooth, affordable transfers between flights and we are delighted to have entered into partnership with easyJet which was an obvious and natural fit for each airline's large and growing networks", Norwegian Air said in a statement to Reuters.
The airline, which has been bolstering its transatlantic charge this year, said it had recruited 400 pilots since the start of the year, and 140 of those have come from rival Ryanair.
According to the Facebook post in which the carrier broke the news, Ryanair "have messed up in the planning of pilot holidays and we're working hard to fix that".More news: PV Sindhu gets better of Okuhara, wins Korean Super Series
Ryanair also said the drastic decision to cancel flights was motivated by its need to improve punctuality, which has fallen from 90% to below 80% in the first two weeks of September.
While the scrapped flights account for only about 2 percent of normal daily operations, the cause was avoidable.
Carole Schofield tweeted: "How the hell do you know if you can get back".
The page will be continually updated throughout the six weeks of mayhem, so it's worth keeping an eye on if you've a later Ryanair flight booked. "Ryanair is not short of pilots - we were able to fully crew our peak summer schedule". Ryanair also said air traffic control strikes and weather disruptions were affecting its performance.
A "line of flight" refers to the schedule of flights an aircraft operates in a day.
12 flights in and out of Dublin are among those affected today.
But it now has a backlog of holidays for pilots and crew members which must be taken before the end of the year in order to switch to a calendar leave year - as required by regulators - from January 2018 onwards.