Ellen, I came across this birth cert of a Margaret McManus but unsure if it is your g grandmother.
Margaritam McManus 13 July 1846 Taunagh,co Sligo father:Jacobi(James)McManus mother:Catherinae Conolly
I know the year is a bit before 1852 but the Irish are known for not giving their exact year of birth.Its just one to keep in mind.
I checked passenger lists for Boston but could find no Margaret. However when I looked at records for Castle Garden,NY I found 12 Margarets between 1846-1872,3 of whom have an age that would fit.The curious thing is that one of them on 29 July 1848,ship Caroline Nesmith,has 3 McManus family travelling together Margaret age 2,Catherine age 14 and Bridget less than 12 months.It is possible that this is Margaret and sisters maybe going to meet their mother in America. Its possible perhaps that the father died in the famine.All supposition but this looks a good match for your Margaret.Also in the 1870 US census there is a Catherine McManus age 62 living in Massachusetts with a daughter Bridget age 22 which would match with the age of baby Bridget on boat in 1848.Maybe try to trace them through Massachusetts records to see if they match.There are a couple of marriages and a couple of deaths that may relate to Catherine(both the mom and sister)which may be worth looking at.Hope this helps,Kim