For Scottish traditional music, I'd like to second the Old Blind Dogs, Tannahil Weavers and Silly Wizard - they all are on high rotation at my place.

For Irish traditional I really enjoy Dervish. Not only are they great musicians, but they are terrific researchers, digging deep into old Sligo tunes. I guess some would argue they are not really traditional, but I also love Solas.

Have you checked out the "NPR Curious Listener's Guide to Celtic Music" by Fiona Ritchie? I bought it as soon as it came out and I have found it to be a fabulous guide. When I first got into Celtic music I was just overwhelmed and didn't know where to start. This book really helped me start building up my collection.