Don't know much about ancestry.com, but I have used familysearch.org, which is the well-known Mormon site.

Even using free resources, the problem with using data that others have researched is knowing how accurate it is, or isn't. My mother has done a lot of genealogical research on her side of the family, and the Irish clan name comes from her father. She has traced it back definitely to at least the same Irish county (Cork) that the O'Callaghan clan came from, although not quite back to the clanlands around the River Blackwater, and even found where to place blame for a change to a variant of the name (the Royal Navy were responsible for that). I found a possible branch of the same family in the Mormon database, and have corresponded with one of them, but whether we are truly kin is hard to say, although the info they posted would extend my mother's work back further, to the 18th rather than 19th century.

Most genealogy that goes back very far depends on family trees compiled by others, however obtained, and in the case of famous people it may be even harder to believe than for us ordinary folks, as they tend to have a priori ideas about who they want to be in their family tree! For example, there is a branch of Callahans in County Waterford (to the immediate East of County Cork) who have British titles of nobility and are intermarried with the British royal family, who claim their family tree goes back to the progenitor of the clan, and it may do, as perhaps even ours does (despite ours mostly being common sailors), but much of their family tree has nothing to back it up, AFAIK.

As for the Mormons, I won't make this a discussion of religion, but their interest in genealogy appears to stem from the quaint practice of posthumously baptising the dead. I assume no actual water is involved! Seems harmless enough, although I know it offends some people that they do this.