There is a Sea, next to Ireland that I never heard of – What a surprise! I had always presumed that only one label existed – the North Channel – but the Straits of Moyle (Sruth na Maoile in Irish and Scottish Gaelic) or Sea of Moyle sits in the North Channel between northeastern County Antrim and the Mull of Kintyre. The distance between the two shores is approximately 12 miles (20 km) at its closest point, and thus it is possible to see across in clear weather conditions.
How do mapmakers decide where a sea ends or begins? It seems that the Moyle Sea is considered to comprise of six islands: Rathlin, Tory, Arran, Islay, Gigha & Colonsay. All have been occupied since the Iron Age and hosted early Christian communities. At least 5 of the islands were within the Dalriada (Dál Riata) over-Kingdom in the 6th to early 7th century, and all were subject to early Viking Raids and then formed part of the Scandinavian Sudreyjar, or “Southern Isles” for the next 400 years.

There is also a trek called the Moyle Way which is a 42km two-day walk that explores the northern-most Glens of Antrim. Following a mixture of forest tracks and remote upland moor, the route includes a visit to the lopes of Trostan, Antrim’s highest summit at 550m.

References
https://en.wikipedia.org/wiki/Straits_of_Moyle
https://wikivividly.com/wiki/Straits_of_Moyle
https://trailhiker.wordpress.com/2014/08/16/moyle-way-from-ballycastle-to-breen-forest
http://www.walkni.com/d/walks/187/A%20Walker’s%20Guide%20to%20the%20Moyle%20Way.pdf