The office of Deemster is a very old one, though the list of names of known individuals to hold the office "only" goes back to the early 1400s or so I think. You can find out a little bit more about them on this link to the site of a firm of Manx lawyers.
http://www.dc.co.im/iom-legal.htm I think the senior Deemster also has some responsibilities in the Manx Parliament and so usually has to be able to speak (or at least read out statements in) Manx gaelic. You can probably find out some more about that if you google against the name of the parliament, "Tynwald"