Doug:
Civil registration of Roman Catholic marriages started in 1864 so we have to work with available church records..
I looked at the subscription site Roots Ireland and focused on Templemore RC records only on the assumption that the family was living in Templemore parish and marriages almost always occurred in the bride's parish.
There were many Treacy marriages in Templemore. In fact, there are too many (eight) Mary Treacy marriages in the 1840s into the 1850s to give you a lead. For Ellen Treacy and Anne (Nancy is a variant of Anne) Treacy there was one possibility for each name in Templemore.
February 19 1855 William Donnellan to Ellen Treacy
April 25 1857 Jeremiah Cormack to Anne Treacy
If you have additional information, please let us know.
Roger McDonnell