If you wish to limit the Curragh to certain civilizations, it is not necessary to create a new technology. You can go to the editor for your game, and under the Edit Rules>Units tab, scroll down to the Curragh. Under the Available to: heading, simply Ctrl-select the seafaring civs that you would like to be able to build the unit while leaving the others blank.
If you would like to add the technology regardless for this unit or any other purpose, there are a few things you will have to add. First, in order to make a technology un-researchable, go to the Civilization Advances tab, Add a new technology, and make sure the Era is set to None. You will then create a new Civilopedia Entry, e.g. TECH_Seafaring. This entry references both the PediaIcons.txt and Civiliopedia.txt files which are found in the Text folder.
If you are adding graphics or units, you will need a new folder in the Conquests>Scenarios folder to store all your new files. The Conquests scenarios that came with the game such as 'Age of Discovery' or 'Middle Ages' are a great reference for how these folders are set up. You will basically need a new folder tree so that it is accessed properly.
First create a folder inside your Civilization III\Conquests\Scenarios\ folder and title it whatever you like (Timerover, for example). Then go inside your editor to the Scenario Properties>Scenario tab. Under the entry of Scenario Search Folders, type the name of your mod (Timerover, in our example). Now, whenever someone opens your game, it will search your mod folder for changes.
Inside the Timerover folder, you will need to create a folder named Art and one named Text, just like in the basic Civilization III folder. You can copy the Text files from the regular Civ III conquest text folders, paste them into your Timerover\Text folder, and then edit them. For the PediaIcons.txt document, scroll down until you see all the other Tech_(name) options. Add an extra line with the name of your tech (such as TECH_Seafaring), and then scroll down further and add another line that reads TECH_Seafaring_LARGE. You will also see how underneath each tech is a path to the folder containing the appropriate picture. You can either use an existing picture, or create your own. Let's say you want to use the nice sailing picture that came with Conquests for simplicity's sake. This picture comes with your game and is located in Conquests\Art\tech chooser\Icons. You just have to set the path in your PediaIcons so it looks something like this:
#TECH_Warrior_Code
Art\tech chooser\Icons\81-Warrior Code-small.pcx
#TECH_Writing
Art\tech chooser\Icons\82-writing-small.pcx
#TECH_Assassination
Art\tech chooser\icons\BloodCult-small.pcx
#TECH_Fascism
Art\tech chooser\Icons\Fascism-small.pcx
#TECH_Seafaring
Art\tech chooser\Icons\Sailing_small.pcx
and then do the same thing for the LARGE Icon.
This gives your game a reference for a picture so it won't crash, but you still need a civilopedia entry. Open up your Timrover\Text\Civilopedia Text file and scroll down until you see the Tech entries near the bottom. You will add an entry:
#TECH_Seafaring
^
^Available only to Seafaring civilizations.
or something along those lines. Then you will add directly below it:
#DESC_TECH_Seafaring
^
^Sea faring was invented by blah blah...
where you can write a long-winded exposition on the history of sailing.
Now that you have these areas covered, there's a few things to do back in your editor. First, under the Edit Rules>Civilization Advances Tab, make sure the 'Cannot be Traded' box is checked. Then, head over to the Civilizations tab and place your new seafaring tech on the list of Free techs for all appropriate civilizations.
