Jim--you would be right if the early published sources used the word BRACKEN. As far as I can see none of them do, they invaribly give BRECKAN which would be the non gaelic speakers pronunciation of the word BREACAN in its various spellings.
Nowhere, in the South of Scotland at least, have I ever heard bracken pronounced breckan and as one brought up among the ootby herds I have had plenty of opportunity of hearing bracken discussed in various dialects from Tweedside to Stran-Na-Gower.
Joe.