Ryanair needs to comply with UK Civil Aviation Authority instructions by 5 pm today, Friday, September 29 or the airline will be fined.
It comes after the CAA on Wednesday accused Ryanair of breaking European Union laws as the email it had sent customers did not sufficiently tell them about their rights.
The airline said it had replied to the CAA, agreeing to meet its requirement for customer clarification on Ryanair's EU261 obligations, which are in line with those requested by the auto.
In addition to making restitution to those affected, the CAA also demanded Ryanair vow to assist any passengers whose schedules had been disrupted as a result of the mass cancellations announced over the preceding two weeks, as well as publicly advising them on how it planned to re-route passengers that would require it.
It agreed that passengers affected by the thousands of flight cancellations do have the right, not just to a refund, or to being rerouted on a Ryanair flight but, in certain circumstances, to a ticket on an alternative airline.
Ryanair's announcement that it is cancelling 18,000 flights between November and March has understandably evoked the ire of the 400,000 customers affected.
The CAA also ordered Ryanair to inform people who "may have chosen an option that was not suitable for them as a result of any misunderstanding of their EU261 rights" were entitled to change their mind if they so wished.
The CAA had given Ryanair until 5pm today to clarify its passengers' rights under the EU261 regulations.
"We apologise again sincerely for the disruption and inconvenience our rostering failure has caused some of our customers".
Ryanair's offer to passengers features several conditions, including assessing the cost of flights on other airlines "on a case by case basis" before bookings are made.
As it did when announced the first round of cancellations, the CEO of Europe's leading low-priced airline, once again called for "heartless apologies" to affected customers.
Ryanair's Chief Marketing Officer Kenny Jacobs said: "We are committed to processing all such claims within 21 days of receipt and hope to have all such claims settled before the end of October".
Brehany did, however, say Ryanair may have to "rebrand and do all sorts of things in order to recover their market share" if it doesn't "get a grip and follow the very robust position offered by the CAA".
Several popular routes used by United Kingdom travellers were hit, such as Stansted to Edinburgh and Glasgow, Gatwick to Belfast, Newcastle to Faro, and Glasgow to Las Palmas.