Is 'deirfiúr' not a feminine noun? I would love to get the gender of words in the hints.
ETA: Sorry, my bad, words starting with 'd' are not lenited. This language is funnier than I remembered.
Words starting with 'd' can be lenited, but when the letters d,n,t,l,s come into contact they prevent lenition. This is called the dentals rule.
Yes, that's true! You will still occasionally see "An bhean dearg" as well, obeying the dentals rule.
Grammar overload? The simple version is don't lenite d,n,t,l,s when they are next to each other.