Curtin and O'Connor families

Hi!

I'm seeking info on my great-grandfather, Thomas Curtin,  

Here is everything that I know about him (not much, unfortunately)::

• Thomas Curtin came to American with his brother, James Curtin, and his half- or step-brother, Michael O'Connor. 
• In the 1900 U.S. census Thomas Curtin stated he was born Sept. 1852 and came to America in around 1870.
• His 1872 U.S. Army enlistment said he was age 21 and was from County Clare.  He enlisted at the same time as a Martin Curtin, age 23, also from County Clare.  They enlisted together in Boston, likely his port of entry to the U.S.
• He is listed as Thomas A. Curtin in several Washington, D.C. City Directories. 
• His death certificate lists his father as James Curtin and his mother as Hannah O'Connor.  I'm not sure if O'Connor is a maiden name or her name from a subsequent marriage - given that Thomas Curtin came to America with a half/step-brother named O'Connor.
• Thomas Curtin was Catholic.  Some of his siblings remained in Ireland, some of whom, according to family stories, became priests/ nuns.  
• I have a photograph of a "Miss Mary Curtin" taken at a photography studio in Limerick. It was probably taken in 1860s judging by the style of clothing, 

.Any assistance would be greatly appreciated!