After I installed MainStage 3 and downloaded all additional content in Mainstage 3,
I am seeing the Irish Fiddle in GarageBand, when I enter "Fiddle" into the search field of the library or go to the "Legacy" section of the sound library:

I have not used MainStage once, I am just using it to download the JamPacks.
Launch MainStage and go to the menu "MainStage 3 > Sound Library > Sound Library Manager".

In the Sound Library Manger select the sounds and loops you want to install.
The Irish Fiddle should be in the "JamPack World". Select the sounds for this JamPack to install.
You may also want to get the Jam Pack Symphony Orchestra, it has a nice collection of strings.

Once the additional sounds and loops have been installed, restart your Mac. In GarageBand go to the Loop browser to reindex all loops.
Then create a new Midi Track and enter "Fiddle" into the search field of the Library. Does the Irish Fiddle appear?