immigration records of my ancestors.

I am trying to find the immigration records of my ancestors.  I know that they came to America during the potato famine but I am not sure where or when they arrived.  I am assuming in Boston since the 1850 US census shows them in Worcester MA.  On the census my 2nd Great Aunt is not listed though her 2 brothers Michael and Martin are?  My 3rd Great Grandfather John Ball would have come over with his wife Margaret Ball (Joyce) and children Michael and Hannah.  Michael would have been a toddler and Hannah a baby. I have been unable to find any records of them in Ireland or MA for that matter.  I am pretty sure they were from Galway or and County Mayo Ireland.  Any tips or help would be greatly appreciated,  Thank you.

Ryan Ball