It is an area of historical and geographic significance, bounded by the Quoile Marshes (now drained, but formerly extensive), the Blackstaff River, the Irish Sea and Strangford Lough. It has an oddly isolated position, virtually cut off from its hinterland, but open to invasion and influence from the east and south.
Anciently it was the name of the Irish district of Leath Cathail. As Ladcathel it was a county of the Earldom of Ulster, and later the name of the former barony of Lecale, which was then split into Lecale Lower and Lecale Upper by 1851.
Leath Cathail is said to consist of the present-day baronies of Lecale Lower and Lecale Upper, and was a subdivision of the ancient kingdom of Ulaid. It gets its name from Cathal, a prince of Ulaid about 700 A.D. who was a descendant of Fiachna, a son of Deaman, a king of Ulaid. Hence Leath Cathail literally means "Cathal's half of Dál Fiatach".
More anciently Leath Cathail was known as Magh Inis, meaning the "island plain", with the name "Isle Lecale" still used in the area. These names reflect that until the first sea barriers and drainage systems where constructed about 200 years ago, that Lecale was almost entirely encircled by Dundrum Bay, Strangford Lough, and the Irish Sea.
The Cenél nÓengusa are noted as being kings of Leath Cathail, with the Ua Flathraoi cited as lords by the 12th century. According to the Annals of the Four Masters, on the day of the Festival of Paul and Peter (29 June), 1147, the Ulaid gave battle to the Cenél nEógain of Tír Eoghain who had pursued them to the shores of Dún Droma (Dundrum Bay), Leath Cathail. The Ulaid suffered a heavy defeat, including the death of Archu Ua Flathraoi, "lord of Leath-Chathail", with the victorious Cenél nEógain plundering Lecale and taken off with Ulidian hostages.