Lemme chime in here... My Dad and I both have a SWK Irish, (although I tend to wear my USA Ireland's Nat. more often.) And I have found that we mostly will wear black with it. I always think the shirt and hose should match, so it's generally black shirt and socks, green flashes if socks are up.

I have worn Oatmeal/beige/stone/tan etc.. shirt and hose too, it goes with damn near everything! I think matching a green shirt with a predominatly green tartan is tricky, since the shades of green may not always compliment each other.

I have worn Bottle (dark) green, which is darker than the tartan, shirt and hose, yellow flashes. I thought it looked good, but the wife said it is possible to have on too much green!

Another combo that worked better was black shirt, bottle hose w/ black flash. Dad has worn a white shirt and green hose on occasion and looked ok.