The Irish airline, Europe's largest by passenger numbers, said the memorandum of understanding could give it potential access to 360,000 tonnes of SAF from 2025 to 2030, a fifth of what it says it needs to reach its 12.5% target.
"I'm not sure we'll get there (12.5%) but by signing up more partnership agreements with Shell, with Neste and with the other fuel suppliers, I think that gives us our best chance of maybe getting to 8, maybe 10, maybe 11," O'Leary told a sustainability conference the airline was hosting on Thursday.
"Who knows, hopefully we will get to 12.5% by 2030. But that will not happen unless we have a dramatic revolution in supply of production of SAFs and availability at our airports."
The Shell deal follows similar agreements Ryanair has struck with Finnish biofuel producer Neste and Austrian oil and gas group OMV.
Sustainable jet fuel generally produces up to 70% less carbon than fossil fuels, offering airlines a way to become greener while continuing to fly, before less carbon-intensive hybrid, electric or hydrogen aeroplane options become available.
O'Leary told an Irish parliamentary committee on Wednesday that he did not expect a hydrogen engine to be ready before 2040 or 2050.
Ryanair has committed to cutting its carbon emissions to net zero by 2050 and said the Shell agreement could save 900,000 tonnes in carbon dioxide emissions, equivalent to over 70,000 flights from Dublin to Milan.
Ryanair operates an average of 3,000 flights per day and plans to grow from a record 168 million passengers this financial year to 225 million by 2026.
Shell aims for renewable fuel to account for 10% of its global jet sales by 2030.
(Editing by Bernadette Baum and Conor Humphries)
By Padraic Halpin and Graham Fahy
