try www.manxnotebook.com/famhist/genealgy/mfests.htm - most Manx went via Liverpool to NewYork

Microfilms of the ships mainifests for various US posts post 1820 are available (try your local FHC or public library) - there is an index being constructed but from those I have transcribed you will find that (a) the handwriting is not always easy to decipher and many mistakes in transcription are possible - I know I've been there ! (b) a Manx accent confused many of those entering names and the written name looks nothing like the more usual name (c) the fact that they are Manx is generally not recorded (you often find as english or possibly Irish) - thus you may find it easier to read the films yourself hoping that family groups have been kept together (not always the case).

Dates of departure can then be found by consulting Liverpool newspapers (not usually on the film) but once you get into Mormon emigration years (post 1840), since they do used Liverpool you might find a ship mentioned in their records (a CD of Mormon emigration data is available - try FHC)