The British Militia was the principal military reserve force of the Kingdom of Great Britain. Militia units were repeatedly raised in Great Britain during the Georgian era for internal security duties and to defend against external invasions. The Militia Act 1757, passed by the Parliament of Great Britain after the outbreak of the Seven Years' War, led to the rapid expansion of the British Militia in order to defend from potential French invasions. In the Kingdom of Ireland, a client state of Great Britain, the equivalent force was the Irish Militia, which saw heavy service in the Irish Rebellion of 1798 alongside British militia units. The existence of militia units in Great Britain and Ireland played an important role in freeing regular troops from the British and Irish establishments for overseas service.

The Irish Army or Irish establishment, in practice called the monarch's "army in Ireland" or "army of Ireland", was the standing army of the Kingdom of Ireland, a client state of England and subsequently of Great Britain. It existed from the early 1660s until merged into the British Army in 1801, and for much of the period was the largest force available to the British monarchy, being substantially larger than the English and Scottish establishments.