The uilleann pipes (IPA: /ˈɪlən/), originally known as the Union pipes, are the characteristic national bagpipe of Ireland. The uilleann pipes bag is inflated by means of a small set of bellows strapped around the waist and the right arm. Found in other European bagpipes (ex. Northumbrian pipes, Scottish smallpipes), the bellows not only relieves the player from the effort needed to blow into a bag to maintain pressure, they also allow relatively dry air to power the reeds, reducing the adverse affects of moisture on tuning and longevity. Some pipers can converse or sing while playing as well.
The uilleann pipes are distinguished from many other forms of bagpipes by their sweet tone and wide range of notes — the chanter has a range of two full octaves, including sharps and flats — together with the unique blend of chanter, drones, and "regulators." The regulators are equipped with closed keys which can be opened by the piper's wrist action enabling the piper to play simple chords, giving a rhythmic and harmonic accompaniment as needed. There are also many ornaments based on multiple or single grace notes. The chanter can also be played staccato by resting the bottom of the chanter on the piper's knee to close off the bottom hole and then open and close only the tone holes required. If one tone hole is closed before the next one is opened, a staccato effect can be created because the sound stops completely when no air can escape at all.
The uilleann pipes have a different harmonic structure, sounding sweeter and quieter than many other bagpipes, such as the Great Irish Warpipes, Great Highland Bagpipes or the Italian Zampognas. The uilleann pipes are often played indoors, and are almost always played sitting down.
The word uilleann comes from the Irish (Gaelic) word uillinn, meaning elbow, emphasizing the use of the elbow when playing the uilleann pipes. However, the pipes were originally called "Union pipes," the first printed instance of this at the end of the 18th century, perhaps to denote the union of the chanter, drones, and regulators. Another theory is that it was played throughout a prototypical full Union of England, Wales, Ireland, and Scotland. This was only realized, however, in 1800, with the Act of Union; the name for the bagpipe slightly precedes this. Alternatively Union pipes were certainly a favourite of the upper classes in Scotland, Ireland and the North-East of England and were fashionable for a time in formal social settings, where the term Union pipes may also originate. 
The term "uilleann pipes" came into use at the beginning of the 20th century. William Henry Grattan Flood, an Irish music scholar, proposed the theory that the name "uilleann" came from the Irish word for "elbow". He cited to this effect William Shakespeare's play The Merchant of Venice published in 1600 (Act IV, sc. I, l. 55) where the expression "woollen pipes" appears. This theory originated in correspondence between two earlier antiquarians, and was adopted as gospel by the Gaelic League. The use of "uilleann" was perhaps also a rebellion against the term "union" with its connotations of English rule. It was however shown by Breandán Breathnach that it would be difficult to explain the Anglicization of the word 'uillin' into 'woollen' before the 16th century (when the instrument did not exist as such) and then its adaptation as 'union' two centuries later. See Folk Music and Dances of Ireland, Cork, The Mercier Press, 1971, p. 77. A much more likely explanation is the fact that many bagpipe bags of that earlier type were made from goatskins which still had the fur attached.
The first bagpipes to be well-attested to for Ireland were similar, if not identical, to the Highland pipes that are now played in Scotland. These are known as the "Great Irish Warpipes". In Irish and Scottish, this instrument was called the píob mhór ("great pipes").
While the warpipe was alive and well upon the battlefields of France, the warpipe had almost disappeared in Ireland (as a result of its outlaw by the English). The union or uilleann pipe required the joining of a bellows under the right arm, which pumped air via a tube to the bagpipe under the left arm, with bellows. The uilleann or union pipes developed around the beginning of the 18th century, the history of which is here depicted in prints of carvings and pictures from contemporary sources. At about the same time the Northumbrian smallpipe was evolving into its modern form, early in the 18th century; a tutor of the 1750s calls this early form of the uilleann pipes the "Pastoral or New bagpipe." The Pastoral pipes were bellows blown and played in either a seated or standing position. The conical bored chanter was played "open," that is, legato, unlike the uilleann pipes, which can also be played "closed," that is, staccato. The early Pastoral pipes had two drones, and later examples had one (or rarely, two) regulator(s). The Pastoral and later flat set Union pipes developed with ideas on the instrument being traded back-and-forth between Ireland, Scotland and England  , around the 18th and early 19th century.
The earliest surviving sets of uilleann pipes date from the second half of the 18th century but it must be said that datings are not definitive. Only recently has scientific attention begun to be paid to the instrument and problems relating to various stages of its development have yet to be resolved.
The instrument most typically is tuned in the key of D, although "flat" sets do exist in other keys, such as C♯, C, B and B♭. These terms only began to be used in the 1970s, when pipemakers began to receive requests for pipes that would be in tune with Generation tin whistles, which are stamped with the key they play in: C, B♭, etc. The chanter length determines the overall tuning; accompanying pieces of the instrument, such as drones and regulators, are tuned to the same key as the chanter. Chanters of around 362mm (14 1/4") length produce a bottom note on or near D above middle C on the piano (where A=440 Hz, i.e. modern "concert pitch"). The modern concert pitch pipes are a relatively recent invention, pioneered by the Taylor brothers, originally of Drogheda, Ireland and later of Philadelphia, Pennsylvania, in the late nineteenth century. Concert pitch pipes typically have wider bores and larger tone holes than the earlier "flat" pitch sets, and as a consequence are a good deal louder, though by no means as loud as the Highland pipes of Scotland. They were developed by the Taylors to meet the requirements of playing in larger venues in the United States; today they are the most common type of uilleann pipes encountered, though many players still prefer the mellower sound of the earlier style narrow-bore pipes, which exist in pitches ranging from D, through C♯, C, and B down to B♭. Pipemakers before the Taylors had, however, built concert pitch pipes using the narrower bores and smaller fingerholes of the flat pipes. Some of these instruments seem to have been designed with lower pitch standards in mind, such as A=415. The Taylors also built many instruments with higher pitch standards in mind, such as the Old Philharmonic pitch of A=453 that was commonplace in late 19th century America.
The D pipes are most commonly used in ensembles, while the flat-pitched pipes are more often used for solo playing — often a fiddler will "tune down" their instrument to play with a piper's flat set, but the inflexibility of other instruments used in Irish music (accordions, flutes, etc.) usually disallows this. It is noteworthy that Irish music was predominately solo music until the late 19th century, when these fixed-pitch instruments began to play more of a role. Like pipe organs, uilleann pipes are not normally tuned to even temperament, but rather to just intonation, so that the chanter and regulators can blend sweetly with the three drones. Equal temperament is almost universal with the fixed pitch instruments used in Irish music, which can clash with the tuning of the pipes.