A Monarch is the person who is supreme ruler over an area. This is commonly a king or queen, but it also extends to an emperor or empress, archduke or archduchess and prince or princess if there is no higher ranking noble. If there are two or more people sharing the throne, then they are joint monarchs.
The Monarch of Ireland
Ireland had various kingdoms, each with their own sovereign. There is also reference in various sources of a High King being selected from amongst these kings. However, there is still debate on whether this High King actually had enough power and control to be considered the monarch of all Ireland.