Such an epic idea...I wish I had better suggestions.
Leather bodice, with corsetish detailing, or straps and buckles, maybe?
Cogs in yer sporran. Whatever a sporran is for, it would do it better with cogs.
I recently stumbled across
this amazing dressmaking blog, written by a girl who believes every steampunk costume, regardless of nation of inspiration, deserves a corset and bustle. Have a look at her Victorian/Japanese hybrid outfit, you might like the idea.
GIANT BRASS CLAYMORE, FTANG FTANG!
Sorry about that. But seriously, you need weapons. The Celts have the best weapons X3