The Irish bouzouki is a development of the Greek bouzouki adapted for Irish traditional and other folk music from the late 1960s onward.
Adoption for "Celtic" music 
The Greek bouzouki, in the newer tetrachordo (four course/eight string, or τετράχορδο) version developed in the mid-twentieth century, was introduced into Irish Traditional Music in the late 1960s by Johnny Moynihan of the popular folk group Sweeney’s Men, and popularized by Andy Irvine and Dónal Lunny in the group Planxty. In a separate but parallel development Alec Finn, later with the Galway-based traditional group De Dannan, obtained a trichordo (three course/six string, or τρίχορδο) Greek bouzouki on his own. With a few exceptions, bouzouki players playing Irish music tend to use the instrument less for virtuoso melodic work and more for chordal and contrapuntal accompaniment for melodies played on other instruments such as the flute or fiddle. Because of this it is common to use matched strings on the two bass courses, tuning to unison pairs in order to enhance the bass response of the instrument.
Almost immediately after the Greek bouzouki's initial introduction new designs built specifically for Irish traditional music were developed. The body was widened and a flat back with straight sides replaced the round, stave-built back of the Greek bouzouki. English builder  Peter Abnett, who was the first instrument-maker to build a uniquely "Irish" bouzouki - for Dónal Lunny in 1970 - developed a hybrid design with a 3-piece dished back and straight sides. All of the initial Irish bouzoukis had flat tops, but within a few years a few luthiers such as Stefan Sobell began experimenting with carved, arched tops, taking their cue from American archtop guitars and mandolins. Even so, today the overwhelming majority of builders continue to opt for flat (or slightly radiused) tops and backs.
The Irish bouzouki has also become integrated into some other western European musical traditions over the past forty years. Popularly used in the music of Asturias, Galicia, Brittany, Spain, and even the Scandinavian countries (in fact, there is even now a new Nordic branch of the instrument, having been modified further to suit the unique requirements of those musics). The instrument's role is usually a combination of interwoven accompaniment (usually a mix of open-string drones, two note intervals, bass lines and countermelody) and melodic play. Instrumental arrangements by musicians such as Ale Möller from Sweden, Jamie McMenemy of the Breton group Kornog, Elias Garcia of the Asturian groups La Tuenda and Llan de Cubel, and Ruben Bada of the Asturian group DRD, typify the complex admixture of melody and chordal accompaniment to be found amongst skilled continental players. It has also become fashionable for some of these musicians to mix instrumental pieces from the Balkans into their material, creating the novelty of western European instruments playing music typically played by Bulgarian/Macedonian tamburas or Greek bouzoukis in their native setting.
By far the most common tuning for the Irish bouzouki is G2 D3 A3 D4 . This was pioneered by Johnny Moynihan (apparently in an attempt to replicate the open, droning sound of Appalachian "clawhammer" banjo) first on the mandolin and then transferred to a Greek bouzouki. It was later picked up by Andy Irvine and Dónal Lunny, and quickly became the next thing to a standard tuning for the 4 course instrument.
Other tunings used, although by a minority of players, are "octave mandolin" tuning G2 D3 A3 E4 , and "Open D" tuning A2 D3 A3 D4 . "Open G" G2 D3 G3 D4 , is used by some players and has proven useful for "bottleneck" slide playing.
Ironically, the GDAD tuning is closer to the D3 A3 D4 tuning of the Greek trichordo bouzouki than is the guitar-like tuning C3 F3 A3 D4 used on the modern Greek tetrachordo, and is particularly well suited to a modal harmonic approach to accompaniment as used in Irish traditional music.
Alec Finn, playing a Greek trichordo bouzouki, uses the traditional D3 A3 D4 tuning with the octave pair on the low D course changed to unison.
Description: Bouzoukis, Octave Mandolins, and Citterns 
Amongst many luthiers and musicians the Irish bouzouki is considered to be part of the mandolin family, but for others this new family of instruments is a separate development. In actuality, the mandolin and lute families are related and the bouzouki is a part of that. At any rate, since the genesis of the Irish bouzouki in the late 1960s, luthiers have incorporated so many aspects of mandolin construction, particularly when building archtop Irish bouzoukis, that for most it is a moot point.
For many builders and players, the terms "bouzouki", "cittern", and "octave mandolin" are more or less synonymous. The name cittern is often applied to instruments of five courses (ten strings), especially those having a scale length between 20 and 22 inches (500mm and 550mm). They are also occasionally called "10 string bouzoukis" when having a longer scale length. The fifth course is usually either a lowest bass course tuned to C2 or D2 on an instrument with a long scale, or a highest treble course tuned to G4 or A4 on a shorter scale. Luthier Stefan Sobell, who coined the term "cittern" for his modern, mandolin-based instruments, originally used the term for short scale instruments irrespective of the number of their strings, but he now applies "cittern" to all 5 course instruments irrespective of scale length, and "octave mandolin" to all 4 course instruments, leaving out bouzouki entirely.
Mandolin-family luthiers producing an octave mandolin are more likely to use mandolin tuning machines and reproduce the details and styling of their American-style carved top mandolins. Some luthiers choose to refer to their clearly bouzouki-style instruments as octave mandolins, or even as mandocellos, despite the GDAD tuning. The octave mandolin is usually regarded as having a shorter scale length than the Irish bouzouki, in the vicinity of 20 to 23 inches (50 to 59cm), while the scale length of the Irish bouzouki most often ranges from 24 to 25 inches (60 to 65cm). Some instruments have scales as long as 26 or even 27 inches (66 to 68cm). These longer-scaled instruments are generally acknowledged to possess greater volume, sustain, and tonal richness but some players find the stretches involved in fingering too difficult and so prefer shorter scale lengths.
There may even be a trend towards calling all medium scale four course instruments "octave mandolins" in spite of their tunings and especially if they have carved/arched tops, as well as applying "Irish bouzouki" to any medium to long scale instruments, especially if they are tuned GDAD and have flat tops. Calling all such five course instruments "citterns," irrespective of their construction and tuning and in spite their very tenuous connection to historical citterns, does also seem be a trend.
See also 
Further reading 
- O'Callanain, Niall (1997). The Irish Bouzouki. United States: Mel Bay Publications. ISBN 0-7866-1595-8. — An instructional guide
- Richards, Tobe A. (2005). The Irish Bouzouki Chord Bible: GDAD Irish Tuning 2,447 Chords. United Kingdom: Cabot Books. ISBN 0-9553944-0-6. — A comprehensive chord dictionary.
- Richards, Tobe A. (2007). The Irish Bouzouki Chord Bible: GDAE Mandolin Tuning 1,728 Chords. United Kingdom: Cabot Books. ISBN 978-1-906207-02-1. — A comprehensive chord dictionary.
- McLeod, Zan (2001). Learn to Play the Irish Bouzouki (DVD). United States: Music Sales Limited. ASIN: B00024ONEI. — A DVD instructional guide
- Loesberg, John (1989). Chords for Mandolin, Irish Banjo, Bouzouki. Rep. of Ireland: Random House. ISBN 0-946005-47-8. — A chord book featuring 20 pages of popular chords.