Louth is the smallest and one of the oldest counties in Ireland.
A 'county' is the area of land needed to keep a 'count' in all his pomp and splendour. It comes from the French word 'Comté', a county, after 'Comte'. The equivalent old English words being 'Earl' and 'Shire. A county is similar in size to but not necessarily the same as a diocese, which is the area overseen by a bishop, the ecclesiastical equivalent of an earl. Each county comprises several 'baronies', these being the area of land required to support a nobleman of the next level down, a baron. County Louth comprises four full baronies and Dundalk's two half-baronies (Ardee, Drogheda, Dundalk Lower and Dundalk Upper, Ferrard and Louth). Being on the coast and with its fertile soils, Louth is a rich land and so the county is very much smaller that the west coast counties, which have much more mountain, rock and bog. Louth might well have become somewhat bigger but Gaelic Irish opposition prevented that.
'Shiring' is the process of dividing a land up into shires. 'Louth' was 'shired' early in the 13th century, at the time of Bad King John, Lord of Ireland. Just before the Anglo-Normans arrived here, the whole of the area which is now in county Louth had become part of the south-central Ulster kingdom of Oirghialla. So the Normans named the county Uriel/Oriel. This is shown in the charters grants when Drogheda was founded as two separate towns, Drogheda-in-Meath and Drogheda-in-Oriel.
The name of the county was soon changed to Louth (originally written 'Luva'). The county takes its name from Louth village, once the cathedral city of the Bishop of Clogher. The name therefore derives, like the village, from the chief Celtic deity Lugh (anciently Lugos), as do many cities and regions on the continent (e.g. Lyon, Lugano, Leiden). Louth was one of the first round of counties founded, on conquered Gaelic territory, by the Anglo-Norman invaders of the late twelfth century. The failure of the Anglo-Normans to complete their conquest, especially of the north and west of Ireland, inclduing most of Oirghialla, left Louth small and precarious, exposed to the depredations of the Gaelic Irish, while it also acted as a garrison and as a stepping-off point for English incursions further into Ulster (of which Louth was then a part). The Anglo-Normans established eleven 'Boroughs' across the county, several of them later becoming major, walled towns and thriving to this day. Ardee, Carlingford and Drogheda - which was two separate towns, in two different counties, until 1412 - and Dundalk are examples of such, while other Anglo-Norman boroughs are now long abandoned and almost entirely forgotten (for example Castlering and Castle Roche).
Louth was fairly thoroughly colonised, and remained, by and large, loyal to the English crown throughout the mediaeval period. On Mayday 1316, Edward de Bruce, less-famous brother of the Scottish king, Robert, crowned himself King of Ireland, in Dundalk, or so it is widely believed. Just over two years later, in October 1318, he was killed by the English at Faughart Hill, nearby. In the later Middle Ages the English power in Ireland retreated further, resulting in the physical delineation of the area that remained under English government control by the erection of a bank, topped by a fence of sharpened stakes (a paling). Most of Louth lay within this retrenched colony, still referred to as 'the Pale'.
Louth stopped being part of Ulster around the year 1600. The Ulster counties only became really settled thereafter and maps of that period show Louth covering varying areas including parts of Meath (Slane and even Navan), south Armagh (across the river from Newry) and with a shared border with Cavan.