The short answer is, not very much.

While the US Civil War was the first time ironclads fought other ironclads, the French had used coastal ironclads (or self-propellered armoured batteries) of the Aetna class 1855 against Russian coastal fortifications during the Crimean War.

The British and the French had commissioned large sea-going ironclad warships before the US Civil War in the form of [HMS Warrior](https://en.wikipedia.org/wiki/HMS_Warrior_(1860)) 1860 and Gloire 1859. Both nations continued to put sea-going and coastal ironclads into the water as the US Civil War raged.

May 1865 the French had 10 sea-going ironclads with another 6 launched but not commissioned yet, plus 13 coastal ironclads. The British had 22 sea-going ironclads (unfortunately I don't have numbers of coastal ironclads for the British at this time).

Both the British and French have several ironclads that were faster, larger, more heavily armoured and equipped with rifled guns capable of penetrating a lot of armour than anything in the CS or US arsenal. Both the CS and US had problems casting larger rifled guns, and the smoothbore Dahlgrens also had problems with cracks and other faults in the casting, causing several of them to explode. It was usually advised to not use a full charge of gunpowder with the Dahlgren guns. The US had only a small handful of rifled guns on ironclads.

Arguably, both the French and the British had several ships that would have been capably of taking on the entire combined might of the CS and US navies on their own since they were faster, had more armour and carried better guns with longer range.

The world did watch the ironclad battles of the US Civil War with interest, but realised they were mostly coastal ironclads and ad-hoc designs fighting around naval landings, bombarding forts and ports and fighting in or against blockades, pretty much like the French coastal ironclads had 1855 in the Crimean War.

Instead it was the [Battle of Lissa](https://en.wikipedia.org/wiki/Battle_of_Lissa_(1866)) 1866, between the Austrian and Italian navies (the Austrians won), the first fleet engagement at open sea between ironclads (12 Italian and 7 Austrian ironclads) that affected the development of ironclads and the thoery of naval warfare. The Austrians, realising that the slow-loading guns of both navies had problems hitting the enemy ships and did little damage when they did went in to ram the Italian ironclads, sinking two of them. Form then on, until the end of ww1, ships were equipped with ram bows and ramming tactics and how to execute or avoid them was studied and trained among the world's navies.

I'd like to argue that the only big development of the US Civil War that made it out to the world was the USS Monitor - many other countries, including Sweden, built Monitors as coastal ironclads and rotating turrets became commonplace among ironclads. The most successful Monitor was probably the Peruvian raised turret (so that the ship could have raised sides to and become a blue water vessel) Monitor [Huascar](https://en.wikipedia.org/wiki/Hu%C3%A1scar_(ironclad)) which almost single-handedly staved off defeat for the Peruvians in the Pacific War 1879-1883 until she was captured by the Chileans.