In Irish music, many of the instruments used are diatonic. In other words, they are not chromatic; it's hard to play in more than two major keys and their related minors. For example, on a standard tin whistle or uilleann pipes, the player can only play in the keys G major or D major, as well as the related modes that have only one or two "sharps." The lowest note played by these instruments is usually D, and this standard has become known among Irish musicians as "concert pitch."
In the 1970's it became fashionable among fiddlers to tune their strings a half-pitch up, so that their D string became an E-flat string. Many fiddlers liked the "brighter" sound.
However, since 2000 it has become increasingly popular to play various instruments at lower pitches. The sound is often mellower" than the normal high-pitched instruments, and lends a different tonality to the tunes, especially when played at a bit of a slower pace. Interestingly, many antique instruments were designed to be played at lower pitches, but went out of fashion for a long time. They have now been revived, and while they are often harder to find than "concert pitch" instruments, they are readily available, albeit for a price.
One of the albums that really sparked this trend of playing at a lower pitch was Kitty Lie Over by Mick O'Brien and Caoimhin O Raghallaigh, on flat pipes and fiddle respectively. Here is the well-known duo playing the second track off that album:
And here is a brother-sister duet playing a couple jigs on a baritone concertina and viola: