changing planes at FCO

I don't think that's enough time. What if your flight to Rome is delayed? With so little time any delay, even a few minutes will make you late to connect. You may very likely have to change gates/terminals to get to Ryan Air and that takes time. For me that short connection time is too stressful.

None needs to ask if these would be separate tickets—there's no option for them to be anything but.

That gap is nowhere near enough time for the connection.

I just flew from FCO today. I am assuming you would be booking these as two separate tickets. I would not risk this itinerary. You might be able to do this IF and only if everything goes smoothly with Delta. If anything goes wrong - a significant delay or, perish the thought, a flight change - you will end up having to buy a new Ryanair ticket - neither airline will take responsibility for your delay.

FCO is a very large airport with two terminals. They seem to be doing some sort of construction that affects some of the walking routes through and between the terminals. This is what you would need to do:

Delta will arrive in the international area of Terminal 3. If you are near the front of the aircraft, you will disembark quickly. Otherwise it could take some time. There’s a bit of a walk to a tram that takes you to passport control. Passport control went very quickly for me - but it could be different if more planes arrive at the same time. You then will be able to leave that general area and find your way to Terminal 1 to check in to your Ryanair flight - only a few minutes if you don’t get lost on the way. Check in, security (the Fast Track Lane didn’t seem much different from the regular lane this morning, but it couldn’t hurt), then another walk to get to the Ryanair gates (not sure exactly where they are, or how far.)

As I said, this is possible, but with a lot of possible pitfalls. Is there a later flight you could book?

I flew NYC-FCO on Delta yesterday and chose to spend a night at a nearby hotel before flying out on a separate ITA ticket to Palermo this morning. To me the slightly higher cost was worth it - less stress, a shower and a decent night’s sleep - not to mention the fabulous Italian dinner I treated myself to!

Be aware of ryanair check-in, boarding time restrictions

The Bag Drop desks close strictly 40 minutes before the scheduled time of departure.If you miss this deadline your booking could be cancelled without refund and you could be prevented from boarding the plane.

It’s important that you make it to your boarding gate in good time, at least 30 minutes before your flight departure time.

Boarding closes 20 minutes before departure. Please have your valid travel document and boarding pass ready at the boarding gate.

https://help.ryanair.com/hc/en-ie/articles/360017824898-Airport-Timeframes

Really it looks like you have only an hour to make the change.

Theoretically your odds of making the connection are on the plus side of zero, but as a practical matter you are more likely to be struck by lightning.

Just realized yet another reason why this won't work. Croatia's entry into the Schengen Zone has been approved, but not enacted.

In this scenario, the traveler would have to go through passport control for entry into Schengen/Italy, maybe change terminals, check-in, go through security, and then passport control to exit Schengen/Italy before being able to get to the gate.

Thanks Renee for the helpful info from your recent experience! It's good to know about the construction affecting the walk between terminals. Given that we have only been to this airport a few times, and not since the pandemic, we have decided not to risk the connection (although honestly its Ryanair - not a lot of money on the line lol). I've done a little more digging and found another cheap flight to the Balkans out of Bologna two days later. So we'll take your advice and treat ourselves to two days (and food!) in a wonderful Italian city before flying out. A nice little addition to our itinerary and hopefully a much more relaxing experience overall :).