The use of the guitar in traditional Irish and Scottish music is a relatively recent practice. There really isn't a truly "traditional" guitar style in Ireland or Scotland that I'm aware of. But many players are now using guitars for both accompaniment and the playing of melodies. I thought I'd write down a few opinions about flatpicking Irish and Scottish music on guitar.
There are many approaches to playing guitar in traditional music. I'd don't feel there is any one correct approach. My personal preference is to play mostly in standard tuning and sometimes dropped D. Many other players prefer open tunings such as DADGAD. Each approach has certain advantages. With open tunings you can get a really full sound, and can often create moving baselines against open ringing strings. Open tunings lend themselves to note combinations that are partial chords. For example, you might have "d" and "a" notes ringing without the "f" or "f#" that would designate a D minor or D major chord. Many players feel that this kind of harmonic ambiguity is inherent in traditional music. It is certainly true that until recently, Irish traditional music was played without any use of harmony.
As mentioned above, I personally prefer standard tuning. I enjoy the chunkier sound of standard chord positions that usually don't ring as much. The feel is a little bit more like a piano player. Piano use is more common in Scottish, Cape Breton and New England music than in Irish. Some folks have been turned off to that sound by some awful piano playing on early traditional Irish music recordings, but when played well it really drives the dance rhythm into your bones. But getting back to the guitar, the chunkier sound feels more dance-like to me. So that's what I aim for. But I certainly enjoy hearing other pickers play with the open tuning sound. It's still possible to use some really ringing chords in standard tuning, depending on the key you're in, and the textural change from chunky to open can sound great. For melody playing, standard tuning is very versatile, and traditional tunes can be played in their standard keys without a capo.
I played American tunes before I tried playing Irish and Scottish music. Most American flatpickers I heard came from a Bluegrass background. Bluegrassers tend to play traditional tunes with an even emphasis on all the notes, and often replace sections of a traditional melody with licks of their own. These licks tend to have shapes that are different from those found in traditional melodies. Traditional players sometimes vary melodies, but the variations tend to be more like what the tune might have sounded like if it had been created on a different day.
When I started playing Irish and Scottish music, I listened to the internal dynamics of the traditional melodies as played on non-fretted instruments. If you listen to fiddlers, accordion players, flute and whistle players you'll hear a varying emphasis on different notes. This is partly the result of the mechanics of the different instruments, including changes in bow or bellows direction and breath. But it's also from a conscious decision to create rhythmic interest and drive. Rhythmic phrasing varies in different regional styles. Paying attention to traditional ways of phrasing can really help in creating a new "traditional" guitar style. The most basic thing you can do to make a "Reel" dance is to imitate a fiddler's shuffle rhythm. If you tap your foot twice per measure, pick harder on the offbeat (foot coming up). If you tap four times per measure, pick harder on beats two and four. It may seem awkward at first, but after a while it becomes second nature. I don't use this all the way through a tune, but in selected places, and these may vary on different days. If you develop the knack for emphasizing any note you choose, you can really make a tune come alive. I find that with varied phrasing, I can make a tune sound good at slower speeds. A reel at 104 with some shuffle emphasis might sound great. The same tune with evenly picked notes might have to be a lot faster to feel right. What this means is that you can play with a good traditional feel without needing the same level of technique (or should I say speed) of a really hot Bluegrass picker.
Another characteristic of traditional Celtic styles is ornamentation. Some of the ornaments used by players of other instruments are difficult or impossible on the guitar. Ornaments are not requirements. Many tunes sound great without any. But a little carefully placed ornamentation can often bring out the flavor of a tune. The easiest ornament on guitar is a single grace note played as a pull-off or hammer-on. For example, a phrase that starts on an "E" note in the key of D might be preceded by an "F#" grace note pulled-off at the second fret of the first string. First-string grace notes are usually the most comfortable but they can be done on any string. In Irish music the single grace note is usually one note in the scale above the melody note. In Scottish music you might grace with a larger interval. For example, grace the open "E" with an "A" at the 5th fret. This can give a bagpipe feel to a tune. An alternate method of playing a grace note is to use adjacent strings with one follow-through motion of the pick. Fret the "F#" note at the 7th fret on the "B" string. Pick down on the "B" string and continue through the open "E" string. Immediately lift your finger from the second string to stop it from ringing. It's the same grace note as above but it has a different feel.
Single-note triplets are used a lot by four string banjo players and by fiddlers. I find them difficult on the guitar at high speeds. I think the lower string tension and higher volume of a banjo make it easier on that instrument. Some guitar players find it easier to play picked triplets with a thinner pick than those favored by a lot of Bluegrassers. Sometimes I use an alternate method of creating single-note triplets where I pick three notes on two strings with only two motions of the pick. Fret the "E" note on the second string. Pick "down" on the 2nd string and continue "down" to pick the first string. Then pick up on the first string. It has a different quality than the usual method and I can do it at higher speeds than regular picking. You can use it with any open string notes. Try it out. It may appeal to you.
When I got interested in Celtic styles, I had never before played a jig. On the guitar, jigs require some decision making about pick direction. With 6 notes in many measures, there are alternate ways of picking. One way is "down up down down up down". This tends to follow the natural rhythm of 6/8 time. But the two down strokes in a row can be difficult at times. For me, doing this constantly tires my hand and makes me tense. So I use this method part of the time and also use "down up down up down up." I try to practice emphasizing different notes regardless of the pick direction. I feel that using both methods in the same tune can sound really good.
I don't often hear flatpickers play airs, marches and strathspeys. All can sound great on guitar. A simple air, played cleanly with accompaniment from another guitar or a harp can sound beautiful. I often play airs with single notes and leave the chords to an accompanying player. But you can also play airs with strummed full or partial chords. Melodies can be in first position or high up the neck. Many approaches are possible. Marches and strathspeys are fun on the guitar and some of these may be easier for players who haven't developed high-speed skills. On the other hand, some strathspeys are a challenge, with 16th notes played up the neck. Hornpipes are usually played slower than reels. That can make them easier. But some also have more complex melodies and many traditional players use more ornamentation on hornpipes. If you learn an Irish hornpipe from sheet music, keep the following in mind: An Irish hornpipe might be written with continuous 8th notes but it is played with note pairs, the first note of which is held longer than the second. I write out hornpipes with dotted 8ths and 16ths but this isn't really the way it sounds. The feeling is somewhere inbetween the two ways of writing (8th-8th and dotted 8th-16th).
Traditional music can be seriously addicting. I hope to hear more players
flatpicking traditional tunes from Ireland, Scotland, America, England
and any other place with music that appeals to them. As the flatpicked
steel-string guitar is a relative newcomer, I suggest that pickers listen
to non-guitarists to learn what the traditional styles are about. Happy