From the NPU site:
The distinctively Irish form of bagpipe, the Union or Uilleann Pipes,
is believed to have originated about the beginning of the 18th Century,
although no exact source of development has been found. It is thought the name
'Union Pipes' refer to the union of sound produced by Chanter, Drones,
and Regulators. Furthermore, the word 'Union' can probably be attributed to
a corruption of the Irish word 'Uilleann' which means elbow, and compliments
the change from mouth blown pipes to bellows driven pipes, the bellows being
powered by the elbow. The present form of the Uilleann Pipes, with the drones
and three regulators, came into being at the beginning of the 19th century.