There is some debate and speculation as to the origins of the Irish Bagpipe. The first bagpipes in Ireland were probably more like the Highland pipes that are now native to Scotland. This would be the ancient Irish pipes, or, what some call the WarPipes. Medieval times may have seen the development of an Irish pipe more like the Scottish Smallpipe, called the Chuisleann. This was a bellows blown bagpipe with a cylindrical bored chanter and 2 or three drones in a common stock.
The current form of the Irish Uilleann Pipes may have been inspired by the Scottish "Pastoral Pipes". The pastoral pipes are bellows blown and played in a seated position. The conical bored chanter is played open along with 3 drones and (as with most examples of the instrument) 1 regulator. I personally believe that the Uilleann Pipes and the Pastoral Pipes may have developed simultaneously, with ideas on the instrument being traded back-and-forth between Ireland and Scotland. The major difference between the 2 being that the Uilleann Pipe chanter is played in a closed, partially staccato style. Whereas the Scottish Pastoral Pipes are played in an open, legato style. This occurred, roughly, around the 18th and early 19th century.
This early form of the Uilleann Pipes was played relatively unchanged until the late 19th century. Early Uilleann Pipes (or "Union Pipes as they were called) were flat pitched. They were usually pitched around B up to perhaps C sharp. Around the turn of the century, pipemakers began to make what are now called "Concert Pitched" Uilleann Pipes. The Concert pitched set were (and still are) pitched in the key of D and are somewhat brighter and louder in tone. The development of the concert pitched Uilleann Pipes is generally attributed to the Taylor brothers of Philadelphia, PA. Again, it is more likely that several versions were developed, simultaneously in Ireland and America, with ideas making their way from one pipemaker to another. Either voluntarily or, more likely, by quiet observation.
A few great makers in the mid 20th century were instrumental in keeping the instrument from dying out due to the modernization of music and the instruments popular music is played on. The 1960's and the 1970's, on to the present, saw a resurgence in popularity of the Uilleann Pipes, thanks to several traditionally based Irish musical groups that had the foresight to record and tour. Also, with great gratitude from all who love the pipes, to the pioneering organizations in Ireland who kept the music and the pipes from becoming an anachronism.