Fathom Fathom A fathom is a unit of length in the imperial and the U.S. customary systems equal to , used especially for measuring the depth of water. There are two yards (6 feet) in an imperial fathom. Originally the span of a man's outstretched arms, the size of a fathom has varied slightly depending on whether it was defined as a thousandth of an (Admiralty) nautical mile or as a multiple of the imperial yard. Formerly, the term was used for any of several units of length varying around . The name derives from the Old English word "fæðm", cognate
Fathom Castle was being built c. 1180-1200, it most probably was a fortified structure occupied by a knight in the service of DeLacy. There is definite mention of Sir Robert Marmion being at a prest of knights before King John in Dublin in 1210. It would appear that this Sir Robert was the knight responsible for Fidun/Fathom Castle. He entered Ireland with Strongbow and other Normans in 1169-1172 as documented, and there is mention of other Marmions, presumably his descendants being 'of Fidun', i.e. Sir Gilbert in 1290 and a William as late as 1305. With the Gaelic resurgence, the 'Fidun' location