Starting out — The "practice set"
Because of the instrument's complexity, beginning uilleann pipers often start out with partial sets known as practice sets.
A practice set consists of only the basic elements of pipe bag, bellows and chanter, with no drones or regulators. The chanter is available in keys ranging from the "concert pitch" D chanter in half-note steps downward to a B♭ chanter, the latter of which regularly is referred to as a "flat set" (as are any sets below the key of D).
In order to play the uilleann pipes effectively, students must learn to pump the bellows steadily while controlling the pressure on the bag and playing the chanter simultaneously. Therefore, beginning students often play on practice sets until they become comfortable with those basic mechanics. Despite their name, however, practice sets are used not only by beginning players but also by some advanced players when they wish to play just the chanter with other musicians, either live or in recording sessions. In these instances, the practice sets can be tuned to equal temperament if needed.
 "Half set"
A half set is the next stage up from a practice set. As with other forms of bagpipes, uilleann pipes use "drones", which are most commonly three pipes accompanying the melody of the chanter with a constant background tonic note. The pipes are generally equipped with three drones: a) the tenor drone—the highest sounding pipe which is pitched the same as the lowest note of the chanter, b) the baritone drone which is pitched one octave below that and c) the bass drone—the lowest sounding pipe, two octaves below the bottom note of the chanter. The Pastoral pipes had four drones, these three plus one more which would play a harmony note at the fourth or fifth interval. These drones are connected to the pipe bag by a "stock". This is an intricately made wooden cylinder tied into the bag (as any other stock) by a thick yarn or hemp thread. The drones connect to the stock, as do the "regulators" (see "Full Set" below). The stock and drones are laid across the right thigh. This is distinct from other forms of bagpipes, in which the drones are usually carried over the shoulder or over the right arm.
The drones can be switched off. This is made possible by a key connected to the stock. The original design of the stock was a hollow cylinder, with two metal tubes running through it to both hold the regulators, and independently supply air to them. Thus the regulators could be played with the drones silenced. In the late 19th century it became more common to build the stock from a solid piece of wood, with 5 holes bored through it end-to-end. This was less susceptible to damage than the earlier design. The piper is also able to switch on and off various drones individually (applying slightly more pressure to the bag and tapping the end of a drone), which is generally used to aid in tuning (a technique used in almost all bagpipes which have drones) or all of them at the same time using this key. This makes the instrument more versatile and usable not only as a half set, but also to allow playing the chanter by itself. The drones use a single-bladed reed (the actual part creating sound), unlike the double reed used in the chanter and the regulators. These drone reeds were generally made from elderberry twigs in the past — cane began to be used in the late 19th century.
 "Full set"
A "full set", as the name implies, is a complete set of uilleann pipes. This would be a half set with the addition of three "regulators". These are three closed pipes, similar to the chanter, held in the stock. Like the drones, they are usually given the terms tenor, baritone, and bass, from smallest to largest. A regulator uses keys (five on the tenor and four on both baritone and bass) to accompany the melody of the chanter; these keys are arranged in rows to give limited two note "chords," or, alternatively, single notes for emphasis on phrases or specific notes. The notes of the regulators, from highest to lowest (given a nominal pitch of D) are as follows: Tenor: C, B, A, G, F#. Baritone: A, G, F#, D. Bass: C, B, A, G. The tenor and baritone regulators fit into the front face of the stock, on top of the drones; the bass regulator is attached to the side of the stock (furthest from the piper), and is of complex construction.
Another method of using the regulators is to play what are referred to as "hand chords": when the melody (usually in a slower piece of music such as an air) is being played on the chanter exclusively with the left hand, the right hand will be free to create more complex chords, using all three regulators at once if so desired. Many airs end a section on a G or A note in the first octave, at which point a piper will often play one of these hand chords for dramatic effect.
The difficulty of playing a melody, pumping the bellows, keeping constant pressure on the bag and playing the regulators at the same time, precludes most pipers from using the regulators much; some pipers have played for years and years yet have little ability to use them. Some pipe makers also add another regulator with one key to play an E (a tone above the chanter's lowest note); this allows a whole tune to be played with the regulators, which was occasionally mentioned in old accounts of pipers. Sometimes this E key is added to the tenor regulator, or, more rarely, the baritone. Another addition is a "double bass" regulator, giving the notes F#, E, D, below the bass regulator. The regulators use the same double-bladed reed as the chanter. A final occasional variant, the three-quarter set, omits the bass regulator. The pipes evolved from one regulator, to two, to three, which became a de facto standard in the early 19th century.
The chanter is the part of the uilleann pipes that is used to play the melody. It has eight finger holes (example given of a D pitched chanter): Bottom D, E♭, E, F♯, G, A, B, C♯, D' (also called "back D"). To achieve the "bottom D" the chanter is lifted off the knee, exposing the exit of the chanter's bore, where the note is produced. The chanter is set on the right knee thus closing off the bottom hole. Many players use a strip of leather placed over the knee, called a "popping strap," which provides for an airtight seal. More rarely, a simple gravity- or spring- operated flap valve attached to the bottom of the chanter achieves the same end. Generally, for all other notes (except for special effects, or to vary the volume and tone) the chanter stays on the knee.
One characteristic of the chanter is that it can produce staccato notes, because the piper seals it off at the bottom; with all of the finger holes closed, the chanter is silenced. This is also necessary for obtaining the second octave; the chanter must be closed and the bag pressure increased, and then fingered notes will sound in the second octave. A great range of different timbres can be achieved by varying the fingering of notes and also raising the chanter off the knee, which gives the uilleann pipes a degree of dynamic range not found in other forms of bagpipes. Pipers who use staccato fingering often are termed "closed-style" pipers. Those who use legato fingering more predominately are referred to as "open-style" pipers. Open piping has historical associations with musicians (often Irish travelling people) who played on the street or outdoors, since the open fingering is somewhat louder, especially with the chanter played off-the-knee (which can, however, lead to faulty pitch with the second octave notes).
A type of simultaneous vibrato and tremolo can be achieved by tapping a finger below the open note hole on the chanter. The bottom note also has two different "modes", namely the "soft D" and the "hard D". The hard bottom D sounds louder and more strident than the soft D and is accomplished by applying slightly more pressure to the bag and flicking a higher note finger as it is sounded. Pipemakers tune the chanter so the hard D is the in-tune note, the soft D usually being slightly flat.
Many chanters are fitted with keys to allow accurate playing of all the semitones of the scale. Four keys will give all the semitones: F natural, G sharp, B flat, C natural. The C natural key is essential for obtaining this note in the second octave, and is the key most commonly fitted. Older chanters usually had another key for producing d3 in the third octave, and often another small key for e3, and another for D#' (as opposed to the E♭ fingerhole, which could be slightly off-pitch). Most uilleann chanters are very responsive to "half-holing" or "sliding", which is the practice of obtaining a note by leaving a fingerhole only half covered. This is why many chanters sold in Ireland are sold without keys. With this technique and some practice, many pipers can accurately play the semi-tones which would otherwise require a chromatic key to be installed. The exception is the C natural, which in the second octave, cannot be cross-fingered or half-holed, and requires the key. This is why it is the most commonly fitted key.
The chanter uses a complex double reed, similar to that of the oboe or bassoon. Unlike most reed instruments, the uilleann pipe reed must be crafted so that it can play two full octaves accurately, without the fine tuning allowed by the use of a player's lips; only bag pressure and fingering patterns can be used to maintain the correct pitch of each note. It is for this reason that making uilleann pipe chanter reeds is a demanding task. Uilleann pipe reeds are also often called "the piper's despair" for the immense difficulty of maintaining, tuning and especially making the double reed of the regulators and, most importantly, the chanter.