Debby,do you have any idea at all where the family may have lived after coming here? I ask because I was looking at census returns for 1850 and found some people that seem to fit the bill but I'm unsure.
1850 US census:
Allegheny Ward 4,Pennsylvania Mary A Johnston age 42 born England Angelina Johnston age 17,born England William Johnston age 8,born England (don't let England put you off as many Irish put England as Ireland part of Britain at the time)
Allegheny Ward 2 Pennsylvania Richard Johnston age 8,born England (this was some sort of asylum-couldn't read what type on census return-mind you it had all children so maybe poor kids went to it)
North Fayette,Allegheny,Pennsylvania John Johnston age 7 living with the Lake family
I wonder if perhaps the father John Johnson/Johnston may have died soon after arriving in the US(passage was very hard)and the mom Mary Anne had to put her children in various places just to survive as she couldn't work and take care of them all at same time.I also noticed that there are marriages for a Angelina,Sophia and Richard in Iowa during the next 20 yrs.As the certs do not say parents these may or may not be related to you.You can view them on www.familysearch.org.Kim