The Bouzouki of course is a Greek instrument. It is one of a family of Lute (al L'Ud) related instruments and traditionally had 3 courses (6 strings), tuned in a variety of ways but commonly DAD.
Modern Greek Bouzoukis have 4 courses (8 strings) and are often tuned to CFAD.
With such an effective long scale accompaning instrument in existence, it was only a matter of time before it was absorbed into the traditional music of other cultures. It was reputedly introduced into Irish music by Johnny Moynihan in the 1960s and later played by Donal Lunny and Andy Irvine.
Originally, it was retuned GDAE - like a mandolin, mandola, tenor banjo, fiddle etc but quickly acquired an open tuning more in keeping with the often modal traditional music. This tuning of GDAD or ADAD is now probably the commonest. When Luthiers started making specfic 'Irish Bouzoukis', they got flat backs and a slightly shorter scale length than the originals.
The Bouzouki may be strung with either unison (both strings of a course the same thickness) or octave (where the 2 strings of the lower two courses are one thick and one thin).
Octave tuning gives a brighter sound, and typical string weights for this are:
|Course||string 1||string 2|
Note that the thinnest string of the octave pair is uppermost i.e. it would be the first to be struck by the plectrum on a normal down stroke.
For Unison Tuning, use 42/42, 36/36, 17/17, 11/11
Warning: Always check that your bouzouki neck can take the tension before changing string weights.