So in the Greenlough parish records I can find the following Mary O'Neill's baptised in the 1850s. I've highlighted the one I believe you've already found from 1852:
- Mary O'Neill - 1851 - Parents: John O'Neill and Rose Slamin (sometimes written Slamen)
- Mary O'Neile - 1852 - Parents: Daniel O'Neill and Nancy O'Neile
- Mary O'Neill - 1855 - Parents: James O'Neill and Peggy McPake (spelling variant of McPeake)
After that there is a gap until the next Mary, born in 1862, probably slightly too young to be your ancestor.
Daniel O'Neill and his wife Nancy O'Neill have come up a few times in the research of myself and the previously mentioned Patrick O'Neill, a co-researcher who is a descendant of the Stafford O'Neills of Ballymacpeake in Greenlough and has done a huge amount of research in the area.
Patrick is a DNA match with some confirmed descendants of Daniel and Nancy through their son John O'Neill, who married a Rose Heverin. The couple then both moved out to New Zealand. I'm therefore fairly certain that either Daniel O'Neill or his wife Nancy are somehow related to the 'Stafford' O'Neills. Of course, O'Neills marrying O'Neills make the precise connection difficult to find, but I believe it's in there.
In terms of your own link, it may be of note that the area that Daniel and Nancy lived in, right by the Dreenan Bridge (number 12 on the map below, taken from the Griffiths valuations) had a huge number of Henry families living there, many the immediate neighbours of the O'Neills. In fact, Henry is one of the most common surnames in the area. There are also a number of confirmed links between other O'Neill families in the area and Henry families. I believe Patrick already has a number of them in his own tree.
What I would suggest is that firstly you contact Patrick (I can pass on his contact to you); he is far more knowlegeable in the Greenlough area than myself. You've mentioned wanting to use DNA, and I find myself wondering whether you could be a match with Patrick, or at the very least share some matches.
Secondly, I would take a look through the other Mary Greenlough births and see if you can rule any of the other births out and narrow it down to Daniel and Nancy's child.
Hope that helps!