Fathom Britain, it can mean the quantity of wood in a pile of any length measuring square in cross section. In Hungary the square fathom ("") is still in use as an unofficial measure of land area, primarily for small lots suitable for construction. 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
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