The only one I can think of right off hand is the assumption that any Celtic goddess with three faces is a Maiden, Mother, Crone goddess and I’ve written that particular pet peeve before. No Celtic goddess is a Maiden, Mother, Crone goddess. They are triple function goddesses of any age they choose to show themselves as. That would include the Triplet of the Morrigan or the Romano-Celtic Tres Madres. The Maiden, Mother, Crone thing is a New Age assumption without understanding. The Celts and the Gaels in particular loved threes. The day is divided in threes, day, night and twilight and many other things including their deities are in threes.
One minor pet peeve is that what is emblematic of Brighid in one country is the same for her in all countries. Scotland’s Brighid views of her traditions and she herself are a lot different over the Irish Sea in Ireland. It can even be different from Highland to Lowland Scotland because the character of the two groups is very different and until modern times they had been isolated from one another. It’s why Lowland Scots speak Scots or Lallans and English and the Highlanders spoke Gaelic and the thought processes for language influence thinking.
For instance, Scots Gaelic has no present tense in its verbs but Irish Gaelic and English, obviously, do have a present tense. The Scots mind set is to use future tense because if you are doing it now, you are doing it in the future. To say something in the present one most use the participle ending –ing on a verb and “be” with it. My Gaelic teacher was of the opinion that because the Scots that got transported to the South were using it that was how it got into the speech of slaves because there was a lot of intertwining of the indentured Scots-Irish servants and slave culture so “I be” was translated into English when they banded against the English land owners here.
I can’t think of other real misconceptions other than regional differences other than ones I’ve already written about in the past.