Custer et al were genocidal criminals, and shamingly most of them (including Custer, whose mother was a Casey if I remember right) were of recent Irish extraction.
They are a classic example of a brutalised population emigrating and themselves becoming brutes.
Some explanations: The Treaty of Limerick between the ambitious princeling William of Orange and his father-in-law James II was signed using the Treaty Stone as a base for leaning the document on. A condition for peace was that the Irish who had sided with James would emigrate en masse. As soon as they had done so, the treaty was broken and Ireland was in for centuries of horror. The "Wild Geese" - the soldiers who left the country - made for France and Spain and were eagerly taken into the armies of those countries, where most were killed within two years.
You'll find a Wikipedia article about the Limerick suburb of Garryowen.
The notorious 7th Cavalry took up the song.
There's an inspiring book about the Ogala Sioux leader Crazy Horse and Custer by Stephen Ambrose - an essential read.