That is always going to be a tricky one, and in the end it may not be possible. You should also bear in mind that the reference to Drogheda on the census may not mean the baptism was in one of the two parishes in the town, but could have been in one of the surrounding rural parishes.
I would suggest that you try to find out everything possible about your Margaret and her family, and look carefully at details such as sponsors at baptisms of her children, witnesses at marriages etc., and places where she lived. At the same time fully investigate the possible families you have identified, look at who the siblings married, where they ended up, and at the same details of sponsors and witnesses. You may find a clue in there somewhere especially if a less common given name turns up, or siblings appear as sponsors or witnesses or as neighbours, but you may spend a lot of time chasing this and come up with nothing definite.
As you have so many census records I am guessing your Margaret ended up either in England or the USA, if it was the latter depending on which state it may be worth getting her death certificate as I have seen these give the mothers full maiden name.
Generally I have found that ages on census records can vary widely, but often the following pattern is followed - ages on the earlier census' tend to be more truthful, and women generally will put themselves down as a few years younger than their true age. Sometimes if there is a significant age gap between husband and wife the ages given may be adjusted to reduce it. Ages on death certificates were often the best guess by the person who reported the death. Ages on marriage certificates when given were just as given by the individual, age 21 or above was often referred to as of full age and the marriage could take place without the fathers approval. Marriages could take place at younger ages but the fathers approval was supposed to be given, generally if this was the case the age would be given correctly on the certificate. If Margaret was born in 1852 then she would only have been 15 at the time of her marriage - would the difference between a 15 year old and a 21 year old have been obvious to the priest?
Hope that helps,
Michael.