This is a description of law enforcement in Northern Ireland and the Republic of Ireland. Before the Republic (then called the Irish Free State) left the union in 1922, one police force — the Royal Irish Constabulary — policed almost the whole island (aside from Dublin, where the Dublin Metropolitan Police were the main force; Belfast, where the Belfast Borough Police were the main force; and the borough of Londonderry, where the Londonderry Borough Police were the main force before merging with the RIC).
The Royal Ulster Constabulary (RUC), renamed the Police Service of Northern Ireland (PSNI) in 2001, is the direct descendants of that force, while a new police force — the Garda Síochána — was set up in the Irish Free State (Republic of Ireland since 1949). Today, due to the sharing of a land border and legislation introduced to both countries after the Belfast Agreement of 1998, there is cooperation between the main police forces of jurisdictions.