County Louth (Irish: Contae Lú) is a county in Ireland. It is in the province of Leinster and is part of the Border Region. It is named after the village of Louth. Louth County Council is the local authority for the county. The population of the county is 122,897 according to the 2016 census.

County Louth is colloquially known as 'the Wee County' as it is Ireland's smallest county by area (826 km² (319 mi²). It is the 18th-largest in terms of population. It is the smallest of Leinster’s 12 counties by size and the sixth-largest by population.

Despite its fairly average total population, Louth is the second most densely populated county in Ireland behind Dublin, and the fourth on the island of Ireland, due to its small size.

County Louth is named after the village of Louth, which in turn is named after Lugh, a god of the ancient Irish. Historically, the placename has had various spellings; "Lugmad", "Lughmhaigh", and "Lughmhadh" (see Historic Names List, for full listing). Lú is the modern simplified spelling.

The county is steeped in myth, legend and history, and is a setting in the Táin Bó Cúailnge. Later it saw the influence of the Vikings as seen in the name of Carlingford Lough. They also established a longphort at Annagassan in the ninth century. At this time Louth consisted of three sub-kingdoms each subject to separate over-kingdoms: Conaille (Ulaidh); Fir Rois (Airgialla); and, the Fir Arda Ciannachta (Midhe). The whole area became part of the O'Carroll Kingdom of Airgialla (Oriel) early in the 12th century.

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