The United Kingdom has traditionally been governed as a unitary state by the Westminster Parliament in London. Instead of adopting a federal model, the UK has relied on gradual devolution to decentralise political power. Devolution in the UK began with the Government of Ireland Act 1914 which granted home rule to Ireland as a constituent country of the former United Kingdom of Great Britain and Ireland. Following the partition of Ireland in 1921 which saw the creation of the sovereign Irish Free State (which eventually evolved into the modern day Republic of Ireland), Northern Ireland retained its devolved government through the Parliament of Northern Ireland, the only part of the UK to have such a body at this time. This body was suspended in 1972 and Northern Ireland was governed by direct rule during the period of conflict known as The Troubles.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What is Ireland Act 1914?
which granted home rule to Ireland as a constituent country of the former United Kingdom of Great Britain and Ireland