One thing to remember is that Ironclads can be upgraded to destroyers but dont take as long to build and they keep their promotions.
If you are playing a Financial civ or have fair amount of income, you can start building Ironclads when you get Steel. Remember, to build destroyers I think you will actually need two different techs before you can build Destroyers, (one to reveal oil on the map, and another to build a well on the oil). So in the time it takes you to discovery the techs you need and get the oil wells hooked up you can have a few ironclads built (with promotions from barracks, aggressive, vassalage, theocracy), then as soon as the oil is hooked up you upgrade a few and have instant destroyers with no long build time.
In my current game, I am going for a conquest victory. All civs are on the same continent so I had not built any navy. I'm at war with Spain and my Cossaks are destroying her Conquistadors but then she starts sending over Caravels and Galleys to destroy my fishing boats. All my costal cities are now in starvation and I have no way to stop her. So, I have no choice but to roll out a couple Ironclads. Wooden ships are no match for Ironclads in battle and I win every fight and get some promotions. However the AI was smart at being able to use the higher movement points of her ships to out run my Ironclads or escape to sea when possible but I just placed my Ironclads on every sea resource and set them to sentry to guard the fishing boats. Problem solved. So at the very least Ironclads are good for guarding your sea resources.
Then as soon as my oil is hooked up I start upgrading them and now I have instant destroyers with promotions.