Recorded as Caslan, Caslin, Caslane, Caslyn, Casillan, but in former times befoe the 17th century as O' Caisealain, this is an Irish surname. It is said to be almost exclusively from the county of Roscommon, and the best explanation we can offer for the meaning is that it is topographical. If so it described either a person who lived by a caislean (castle) or by a caislin, a small stream. The literal meaning is probably "The male descendant of the person who lived by a castle (or small stream)" but given the variation in Gaelic interpretations, no doubt other explanations are possible. The dictionary of Irish surnames offers no explanation at all. This would suggest that either they dont know, or they dont like to give an indefinate answer. It is rare for an Irish surname not to be a true patronymic, and even rarer for it to be "residential." This may suggest that the ultimate origin perhaps seven hundred years ago is not Gaelic, but Norman-French. The surname appears in the "List of famine Immigrant" (1846 - 1851) with sample recordings including John Caslin or Caslyn aged eighteen, who left Liverpool on the ship Southerner, bound for New York on April 10th 1847, and Margaret Caslan, aged twenty, who left the same port only two days later, also bound for New York, but on the ship England.