Hello,
I don't have the specific information you are looking for, but just wanted to put a post out here regarding McTiernans - I'm a Tierney, and my great-grandfather Michael Tierney (b 1858) emigrated from Ireland to NYC about 1880. His parents were John Tierney and Margaret Murphy according to his death certificate.
A tenuous family story has him coming from Roscommon, but no proof of that as of yet. My Family Tree DNA test links me to the McTiernans, and my closest matches fall in a group listed on the McTiernan.com site that appear to be from around the area in Leitrim you mention.
I'm trying to find some connections and definitive proof of where that part of my family emigrated from - after some good success over here in NY and getting my DNA results, I've been banging away at the brick wall with little additional success.
Right now I am trying to figure out if there were other Tierneys/McTiernan relatives in NYC at the time of my GGF by going through NYC Directories - there appear to be some people at the same or similar addresses as my known family that I plan to track.
If you ever would like to compare notes and trees, please let me know! I'd be glad to share anything I have. I have also tested my DNA at 23andme if you happen to be on there.
I can be reached via this site and my blog - I've included a link to some posts on my great-grandfather Michael and the contact form for me below. (Better than posting my email address here.)
Contact Form: http://currach.johnjtierney.com/contact/
Michael Tierney blog post: http://currach.johnjtierney.com/2010/10/michael-tierney-ny-policeman-part-1/
Best of luck in your searches!
John Tierney