From 'Bagpipes of Maine' by Tom Seymour

It’s doubtful that anyone seeing the word “Uilleann” for the first time could pronounce it correctly. Phonetically, it would be presented as “Ill-in.” And Uilleann in Gaelic refers to “elbow.” Thus Uilleann pipes are “elbow pipes,” meaning that the elbow works the bellows that supply air to the bag.

Uilleann pipes are a purely Irish form of bagpipes. These pipes came into being in the early 1700s in Ireland and other parts of Britain. The current form took another 50 years to develop. 
Uilleann pipes are the most intricate and capable form of bagpipes, with a chanter that can produce two full octaves, something unique to this advanced of pipes. 

The heart of an Uilleann pipe is the chanter and the heart of the chanter is the reed. These are similar to oboe reeds, two-sided and made of cane. Chanter reeds are the bugaboo of beginning pipers. These reeds vary with changes in humidity. Some pipers, the author included, run a home humidifier in winter to keep pipes in the playable range. 

Uilleann pipes are typical of later types of pipes in that rather than being mouth-blown, they utilize a bellows to pump dry air into the bag. Highland pipes and other mouth-blown pipes must use fairly stiff reeds in order for the reeds to survive the deleterious effects of moisture delivered by human breath.
 Uilleann pipes, being “dry” pipes, can make good use of finer, thinner reeds and that in turn enables the chanter to produce a wider range of tones, including some quite intrusive and also some very soft and mellow sounds not possible with mouth-blown pipes.  Uilleann pipes incorporate something called “regulators,” devices that sit alongside the drones in the common stock. A full set of Uilleann pipes includes tenor, baritone and bass regulators. Regulators have keys, the same as other keyed wind instruments. These are played so as to introduce harmonies into the tunes and also, to add a wide variety of corresponding rhythms to a tune. 

While a superb solo instrument, Uilleann pipes, like Scottish smallpipes, work well in concert with other instruments such as fiddle, whistle, Irish flute, bouzouki (a Greek stringed instrument that the Irish have “appropriated” to their use, especially on fast jigs and reels) and guitar. Irish musical groups often have rhythmic support from a bohdran, an Irish hand drum played by a two-ended beater, or “tipper.” 

 Uilleann pipes were widely played not only in their native land, but also in America during the early 20th century. But like Scottish smallpipes, Uilleann pipes fell into a long period of disuse. And also like Scottish smallpipes, Uilleann pipes have experienced a worldwide rebirth. Uilleann pipes require much care to detail and also, careful attention to the overall sound. Try squeezing the bag, working the bellows, fingering the chanter and in addition to all that, using the heel of the palm of the lower hand on the bottom of the chanter to play harmonies and cordwork on the regulators. It seems an impossible task. It’s tough and time consuming, but when it all comes together any difficulties or reed problems are quickly forgotten. Uilleann pipes can play anything from snappy jigs, compelling reels and the most haunting, heart-rending airs of any instrument.
It’s always best to hear any instrument live, but thankfully for we here in Maine, some excellent recordings exist of all the different pipes, including Uilleann pipes. Also, different Irish folk bands feature Uilleann pipes along with the other instruments. Names like The Chieftains, Battlefield Band and Planxty roll quickly off the tongue when thinking of Irish bands with excellent Uilleann piping.
Na Piobairi Uilleann, a worldwide Uilleann piping organization based in Dublin, Ireland, offers memberships to pipers from around the world. Their excellent website features categories such as piping instruction videos by world-famous pipers, used pipes, a tune library and much more.  Anyone who has not heard the sound of Uilleann pipes should visit the NPU website and prepare to be amazed.