Currently, if you have a .Net class you want to access via IronRuby, and you have an indexer there (like myObject), you won't be able to use the indexer with brackets  from IronRuby. You'll get an exception.
Use get_Item(index) instead (pay attention to casing).
my_dataset.Tables # <-- This won't work
my_dataset.Tables.get_Item(0) # <-- This will work great!
All the best,