The Triangulum Galaxy (M33, or NGC 598) is the next closest spiral galaxy to the Milky Way Galaxy beyond the Andromeda Galaxy, located some 2,600,000 light years from the Alpha Quadrant at its closest point.
At some point prior to the bombardment of Iconia, a sect of Iconians settled in the Gamma Quadrant of the Triangulum Galaxy. (Star Trek: Helen)
In 2383, the Triangulans came from the Triangulum Galaxy to invade the Milky Way Galaxy. They encountered the Baudelaires, then attacked the Federation, the Klingons, the Romulans, the Cardassians, the Dominion, and even remnants of the Borg. (The Devastating Discoveries)
In 2414, the USS Helen was transported to the Triangulum Galaxy through a destabilized wormhole. (Star Trek: Helen)