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Did they arrive in New York? If yes, then use Steve Morse's OneStep Search Engines at htp://www.stevemorse.org/ Steve has 2 OneSteps to search 2 different Castle Garden online databases
I should have added that Castle Garden existed as a processing station for immigrants to New York between 1855 and 1890. There is no separate set of records for Castle Garden, as there is for Ellis Island.
Passenger lists for the Castle Garden time period
are available on microfilm so if you know a date of arrival or a ship name you can order the microfilm OR have NARA do a lookup for you OR you can use Steve Morse's ONE-STEP search engines.
There are two One-Steps -- one searches the free castlegarden.org website (CAVEAT: It contains ships going to ports other than NY but they are not noted as such), one searches Ancestry.com's pay-to-view database (which contains images taken directly from the NARA microfilms and consists only of ships to New York)
To check the complete list of film numbers (NARA or FHC) for ships going to New York after 1820, find your date
(year, month, day if known) and write down the NARA or Family History Centre film number you need from the list at
http://olivetreegenealogy.com/ships/filmnos_newyork.shtmlFor more help with this time period see
"Castle Garden:Ships Passenger Lists to New York 1 Aug.1855 - 18 Apr. 1890" at
http://www.rootsweb.com/~ote/ships/castle-garden.htm