Fenix 5s Ironman?

I'm not a triathlete, and I have the F5 rather than the F5S, so I'm not speaking from direct experience. But I do know that I would be gutted if I did an Ironman, and I missed recording the last hour.

And that's the risk you run when you do an event that is close to the stated life of a battery. Battery life can vary depending on the age of the device, and the specific use conditions.

To maximize battery life during an Ironman, you may want to consider the following:

* Turn off the backlight.
* Eliminate any CIQ datafields.
* Turn off navigation features (eg. heading bug, map, etc).
* Use a chest strap HRM, rather than the on-board OHR,
* Use the Ultatrac GPS setting for the bike leg (where you are probably using HR or power to pace yourself, rather than speed). And perhaps use a speed sensor?
* If during your practice events, battery life looks like it may be an issue, consider rigging some sort of lipstick powerbank charger on your bike.

Battery life is always expressed as 'up tp' for a reason. And that's usually because the reality is often much less. I would suggest that unless you can complete the Ironman in 10hrs or better, you will more likely than not find the device will 'die' before the end.

It's up to 14hours with standard/factory(OHR included, GPS only) settings. If you turn off some features and use chest strap you should get 14hours for sure.
I suggest that you install battery widget and monitor it before and after activity so you can get some perception.