After the "Papist Revolt" (Stewart Uprising) in which many Irish (Protestant and Catholic) participated, Irish Catholics were prohibited from carrying weapons, so they further developed the shillelagh.

From Wikipedia:

Originally known in Irish as the bata, or stick, the name shillelagh is an Anglophone corruption (phonological adaptation) of the Irish sail éille and appears to have been associated with the name of the village and barony of Shillelagh, County Wicklow The shillelagh was originally used for settling disputes in a gentlemanly manner — like a duel with pistols or swords. Modern practitioners of bataireacht study the use of the shillelagh for self-defense and as a martial art. Of the practice, researcher J. W. Hurley writes:


Methods of shillelagh fighting have evolved over a period of thousands of years, from the spear, staff, axe and sword fighting of the Irish. There is some evidence which suggests that the use of Irish stick weapons may have evolved in a progression from a reliance on long spears and wattles, to shorter spears and wattles, to the shillelagh, alpeen, blackthorn (walking-stick) and short cudgel. By the 19th century Irish shillelagh-fighting had evolved into a practice which involved the use of three basic types of weapons, sticks which were long, medium or short in length.[5]