The Uilleann Pipes are a form of the bagpipe that is uniquely distinguished to Ireland testified to in the fifth century Brehon Laws. They are probably the most complicated of bagpipes that have arisen in Europe. The name "Uilleann" is the Irish Language word for elbow. Sometimes they, and other bellows blown pipes, are called "elbow pipes".
The heart of the Uilleann Pipes is the chanter. The Uilleann Pipes have a "conical bored" chanter. That is to say the bore tapers from a narrow "throat" on the reed side to a wider end. Eight finger holes of various sizes and spacing are drilled in a straight line along the length of the chanter to produce different musical tones. They are covered by the thumb and three fingers of the left hand and four fingers of the right hand. The Uilleann Pipes are unique among bagpipes in that they can play two octaves (as opposed to the Scottish Highland Pipes that play one octave). They will also play staccato notes. Both characteristics of second octave and staccato notes are possible, primarily due to the chanter being played with the large end closed against a leather pad situated on the player's right thigh. The second octave is also possible because of the nature of the bore and reed. The Uilleann Pipes chanter has a longer and narrower bore than most other bagpipes. That, and the fact that it is bellows blown, (also called "dry reed" , "cold wind" or "dry wind") allows for a reed that is longer and trimmed somewhat thinner than most bagpipes chanter reeds. This is why the Chanter reed on the Uilleann Pipes can cause the player so many problems. They are much more sensitive to weather extremes than many other woodwind musical instrument reeds. Many pipers prefer to make their own reeds because of this. Making reeds is a worthwhile goal of any piper. Reed making is often unique to a particular chanter and no two chanters are often alike in the reeding characteristics. Even two chanters from the same maker.
The second major portion of the Uilleann Pipes is the drones. They provide a harmonic "bed" to compliment the chanter. They are fundamentally cylindrical in bore, although most sets vary in bore width in one way or another. The drones are driven by "single reeds". This is usually a small piece of cylindrical reed cane that is cut partially across and split back to form a tongue. Many wonderful things are being done in modern pipes in regard to composite and synthetic drone reeds. Most often, composite and synthetic drone reeds are more dependable than their cane counterparts and require less air to play.
The modern Uilleann Pipes usually have three drones that are arranged in a common stock, parallel to each other. The shortest drone (tenor) plays a D above middle C (the root tone of the D pitched chanter). The middle drone (baritone) plays a D one octave below that. And, the bass drone plays a D an octave below the baritone drone. The drones have a key in the stock attached to a valve which can start or stop the airflow to the drones. This can be used to nice effect in the music. It also allows the drones to start at playing pressure. There is no ramp-up to the drone note as in pipes with no drone valve key. The low, mellow sound of the Uilleann Pipe's drones is what attracts many people to the instrument.
The third important part of the full set of Uilleann Pipes is those daunting regulators. (I don't know where the term "Regulators" comes from). The regulators are (on a modern full set) three conical bored pipes, set side by side in the same stock as the drones. The regulators are keyed and have a stop on the end. Because the keys are spring loaded shut and because of the end stop, the regulators only sound when a key is depressed. The regulators are fitted with a double reed, very much like the chanter's double reed. Since the regulators don't play a second octave, they generally give less trouble than the chanter. They are also easier to tune, overall, because of "tuning rushes" that extend the length of the bore. If necessary, pieces of material can be attached to the rush wire to tune selected notes on the regulator. I like "tack putty" for that purpose. The keys on the regulators are arranged in rows of three that match in length when the regulators are set into the main stock. With the player in a seated position, the regulators extend across the player's right thigh. In this position the regulators pass under the right hand as it is playing the chanter. The regulator keys are depressed with the heel of the right hand, thus allowing for a simple chordal accompaniment to the chanter and drones. In certain chanter passages, the player may also reach around and play the regulators with the fingertips of the right hand.
The bag on the Uilleann Pipes is fairly straightforward. It is simply a reservoir of air made, most typically, of leather. Most bags of leather are made of chrome tanned leather. These sometimes need treatment with oils or other concoctions to be airtight. "Elk tanned" cowhide needs no treatment. Some bags are made of synthetic materials, as well. Synthetic bags can be surprisingly good and easy to construct. They can be a good bag for beginning or hobbyist pipe makers. The outlet tube of the bellows leads into a stock on the bag where there is a valve to check the airflow in one direction into the bag.
The bellows consist of two wooden paddles (often called "cheeks") that are fitted with a piece of leather around the edge called the "gusset". A valve on the outside cheek brings air in, on the outstroke, and directs it one way out of the bellows, on the instroke. A belt is fitted to the inside cheek to go around the payer's lower rib cage. A smaller belt is attached to the outside cheek to clasp around the player's arm, just above the elbow.