Brendan is Irish from Bréanainn derived from a Celtic element meaning 'prince' - in medieval Latin (and thus birth/baptismal records which were recorded by the church in Latin) the form was Brendanus which led to Brendan in English use and the modern Gaelic form Breandán. Similar Celtic elements led Gaelic students to theorise alternate meanings of 'stinking hair' or 'dweller by the beacon' but they can't back that up through records or use. What meant 'raven' was the Welsh bran as in Branwen...close but no cigar.
Where anyone gets the idea that they know the meaning of Deirdre is beyond me. Scholars have looked up and down and can only come up with best guesses - to quote Oxford's reference, 'of uncertain derivation; the earliest Celtic forms are very variable'. The 'sorrows' meaning is an associative one, tragic heroine Deirdre, sometimes taken of symbolic of Ireland under English rule, has been wholly responsible for that one. Where 'woman' comes from is quite the mystery - it's Bean in Irish. There was a rather negative term based on Middle Irish diul/del which meant, well,...'suck-giver'. They got that from the Latin influence though so really can't be worked up here. The *most likely* meaning is that it came from the Irish deardan meaning 'storm' and thus interpreted as anything from 'stormy, tempestuous' to 'raging', depending on how much the interpreter likes the name ;o)
How to avoid the confusion - my advice would be to avoid all websites and books that link the words 'baby' and 'names'. Serious scholars study names and research their true meaning based on evidence. Babies have little to nothing to do with it and those that cater to the newest generation of baby-namers aren't interested in scholarship ;o)