All you need are:
-Visual Studio. I recommend 2008 or 2010.
-Like you said, IronRuby.
-As far as core classes go, all you need are the classes from the RPG module.
-save_data and load_data are not native methods inside Ruby, they are native methods within RMXP. So you'll have to create these yourself.
Or if you wanted to save some troubles with IronRuby, since at times it can be difficult to work with. Especially if you have IronRuby that has a different Ruby version than RMXP. It won't save the data properly and RMXP won't be able to read it. If you have the slightest clue on how to write your own file format, you'd be good to go. Just read/write the files as binary files. For example, the first couple of bytes within the file are the file header. Then what you can do is read the file until the end.
Write File Header
For Loop Items
-Write Byte marking Item (1 byte)
-Write Item Id (probably 4 bytes)
-Write Item Name Length (4 bytes)
-Write Item Name (X Bytes)
etc...
Then you can simply read it back the same way.
Open File, Check Header
Read a single byte until the end of file
If byte matches the byte marked as Item
-Read 4 Bytes for the id
-Read 4 bytes for the string length
-Read above result to get the item name
-etc...