Without an initial, it would be hard to establish exactly which "Thomas" we are talking about. There are several listed in lists of Gunmakers of the era. Your blunderbus however bears English Proof Marks similar to that posted here at post #5
http://www.vikingsword.com/vb/showthread.php?t=19534.. The marks in question were in use from 1750 to 1800.
MF2878 is an Irish registration number per their current Laws
Stu