The low-cost carrier is seeking a new head of communications - a role famously dubbed 'the worst job in PR'.
Kiely said he is leaving next Spring to spend more time with his young family before seeking a new challenge.
"It’s been an amazing experience and I’ve got more out of this job than I could have ever hoped for, so it’s the best time to move on," Kiely told PRWeek.
"It was billed as the worst PR job, but I think anyone in the industry knows it’s the opposite and how challenging and rewarding it can be. PR still plays such a prominent role within the business, but the job itself has also changed a lot, as has the airline."
Kiely’s position spans media relations, social media and content. After landing the role in 2013, he helped establish and grow Ryanair’s social channels to more than five million followers. The business now has dedicated social and customer services teams and earlier this year launched a travel content hub ‘Try Somewhere New’, which features thousands of bespoke guides and videos in seven languages.
Under Kiely’s watch, the airline has transformed its approach to media relations from being 'brash and abrasive' to building a strong rapport with journalists.
This has coincided with a charm offensive and more thoughtful approach to customer relations backed by investment in the in-house communications team, which has grown from two members in 2012 to 15 today –across PR, social and content.
During Kiely’s time in the job, Ryanair’s airline traffic has almost doubled to about 140m annual customers flying between 37 countries, which is about 400,000 passengers a day. Ryanair is Europe’s largest airline in terms of international passengers.
Kiely told PRWeek he would be assisting with the recruitment process for his replacement "doing my best Nick Hewer impression".
He added: "I hope to take some time off to spend with my young family and very understanding wife, before finding my next challenge and seeing where my journey next takes me – always on the lowest fares, of course."