Where Thomas was born..... Absence of his age or mother's name on his marriage record makes that very difficult to pin down, I'm afraid. His father's occupation Farmer could mean he was born in any part of Ireland (or Britain).
Any idea of his approx. age on marriage - or even death?
If he was baptised Church of Ireland, the record may no longer exist (destruction of Four Courts, 1922). Or if record did survive, may not be online.
You could possibly try the RCB Library: https://www.ireland.anglican.org/about/rcb-library/
Linen Hall Barracks is shown on the early 1900's map here: http://maps.osi.ie/publicviewer/#V2,715027,734853,12,9 (You can zoom out, etc).
The 17th Regiment.... Google points to 17th Regiment of Foot, named the Leicestershire Regt from 1881. Unless SKS with military history expertise sees your post here & can assist, I'd suggest you try a military history site. I can find very little on the history of the Linenhall when it was used (temporaily) as a barracks.
Have you tried for Thomas's army record? I'm no expert at all but some showed their birth parish.
On another track, have you tried tracing Catherine's father, David? He was also a soldier.....