The fiddle-playing, boozing and occasionally forward-thinking folk outfits may get most of the attention, but at the core of so much of the past few decades of the music of Ireland is that simplest, most famous yet most overlooked of instruments: the six-string acoustic guitar.
Admittedly, outside of maybe U2's The Edge the list of Emerald Isle guitar heroes is almost a non-existent one (and even calling Edge a "guitar hero" might be a stretch), but in the context of traditionally-rooted Celtic music, the guitar has never had much of a role as a purely lead instrument. Most of this has to do with the instrument's relatively recent addition to the typical arrangement: when an entire culture has been writing music in a certain fashion for the better part of a thousand years, it's not unthinkable that fifty years wouldn't be enough of an acclimation period. For purposes of comparison, The Chieftains, easily the most successful and influential traditional Irish group of all time, have avoided employing a regular guitarist entirely for their 47 years together.
Over the course of its short cultural life, the worlds of Irish and Celtic music have instead bent the guitar to fit the style rather than bend the style to the guitar. The result is a familiar sound, yet a wholly unique manner of creating it – no small feat considering the somewhat limited range of the genre as a whole. Most immediately, the specific sound often equated with the style stems not just from the commonality to the arrangements, but also from most tunes being written in the keys of D major and G major. Rather than being some kind of genre-wide conspiracy, this consistency of key is actually a product of the instruments that predate the guitar within the Irish school of songwriting. The Celtic harp, tin whistle and Uilleann pipes, long used as lead instruments, are manufactured in such a way that prohibits tuning or key adjustment; with the lynchpin instruments stuck in one key, and those keys most commonly being D and G, it makes sense the guitar arrangements would follow suit.
Perhaps to make it easier to play along, or perhaps to help advance the cause of jokes about the Irish, alternate tunings are often employed to create a more robust rhythm strumming and open up a new realm of chord possibility. Most commonly, the DADGAD tuning is used (especially in live settings) due to ease of access to both the D major and G major scales, and also the A major scale in more adventurous scenarios. The conventional dropped-D tuning is not uncommon either, as this keeps the lower-octave D handy in a manner that makes chord formation as easy as one finger.
An important adjustment to playing Celtic music is the genre's liberal use of the term "major" when describing scales. While the same scalar intervals apply regardless of song or instrument, a large majority of this type of music actually ignores the third note of the scale. For example, a song may be referred to as being in D major, but in actuality is written in a limited pentatonic D scale consisting of the notes D, E, G, A, B, and C#. This certainly does not apply to every piece of music of the genre, but creates a large part of that vaguely uptempo feel so common to Irish music.
A great example of this is The Dubliners' arrangement of the traditional "Whiskey in the Jar," later made famous when covered by Thin Lizzy (and in turn covered again by Metallica). Note how singer/guitarist Sean Cannon's bouncy strum attack acts turns the guitar into both a rhythm instrument and a percussion instrument in the absence of the bass/drum combination so common to Western music:
Beyond the tonal ambiguity and altered tunings, the largest stylistic decision facing the budding Celtic guitarist is the choice between flatpicking and fingerpicking. Given the generally rhythmic, strummed nature of these guitar parts, it may seem like an obvious choice to choose flatpicking. However, a large number of possibilities for rearrangement exist to turn traditional harp-and-flute or guitar-and-violin duets into solo, classically-styled pieces and for these scenarios where two instruments must be merged into one, fingerpicking clearly wins out. And for those somewhat rare situations where the guitar is given a lead section or arrangement, the tendency of the style is to rely as much on left-hand pull-offs and hammer-downs as on right hand string work, meaning even fingerpickers will find themselves dealing with usually pick-based tricks like crosspicking parallel fourth jumps.
As an example, note guitarist Pat Kirtley in the video below using fingerpicking to create the illusion of multiple guitars, even while playing an unaccompanied solo arrangement of two traditional songs: "Moran's Return," which originated as a fiddle song in Ireland sometime in the 1840's, and "South Wind," which dates back to the late 18th century. Note also how "Moran's Return" is in F major, yet Kirtley retains the Celtic feel less by what he plays and more by how he plays it:
It's a strange relationship traditional Irish music has with the guitar, considering the wide reach of both guitar-based music and Irish culture as a whole. By the same token, some traditions defy innovation, and despite the best efforts of the Pogues and Dropkick Murphys of the world, some music was just meant to sound a certain way.
