I believe that the Irish Chef,Paul Rakin,(whose soda bread we buy in Sainsburys) uses a dark rye flour,which gives a dark nutty soda bread.
How easy this flour is to obtain I don't know.
Thanks for your comments but i think i have it figured out. It seems the trick is to melt some butter in a saucepan until it turns brown and add it to the mix. This, i think, gives it the nutty taste. Will know next time i make it.