"I do the difficult question."
Translation:Déanaim an cheist dheacair.
"deacair" is an adjective. As "ceist" is a feminine noun (you can tell, because it is lenited after "an"), and attributive adjectives agree in case, number and gender, the adjective is lenited to "dheacair".
Because "ceist" is feminine, if "deacair" was applied to the genitive form of "ceist", you would use "deacra":
"freagra na ceiste" "the question's answer" or "the answer of the question"
"freagra na ceiste deacra" - "the difficult question's answer" or "the answer of the difficult question"
If the noun was masculine, the genitive form of the adjective would be "dheacair" - "Teideal an leabhar dheacair" - "the difficult book's title" or "the title of the difficult book".
"slender consonant" only come into play for plurals. "Dheacair" is lenited because "ceist" is feminine.
Nouns that end in slender consonants are usually feminine, but it's the feminine gender that causes the lenition in the attributive adjective, not the slender consonant. A feminine noun like "teanga" will also have a lenited adjective - "teanga dheacair".
can they make it any more complicated ?!
That's a rhetorical question, right? Of course they can!
If you look at the Grammar page on teanglann.ie for any adjective, you'll see that the form that the adjective takes after a plural noun depends on whether the noun ends in a slender consonant (not a slender vowel) in the nominative case, and on whether the noun has a "strong plural" or a "weak plural" in the genitive.
Teangacha deacra, blianta deacra, cluichí deacra.
There aren't any exceptions there - none of "teangacha", "blianta" or "cluichí" end in a slender consonant (they all end in vowels, two broad, one slender)
"na leabhair dheacra", "na capaill dheacra", "na captaein" dheacra"
Oh, I see. I guess I shouldn't have done the lenition exercises by learning each word by heart. So step 1 is to remember whether a noun is masculine or feminine. It's useful to learn each noun immediately together with it's direct article. The rules for doing that are listed neatly here: https://thegeekygaeilgeoir.wordpress.com/2017/08/28/making-sense-of-irish-gender/
I guess that the syntax described in http://www.teanglann.ie/en/fgb/deacair
covers these cases:
the cover of the difficult book
the answer of the difficult question
the cover of the difficult books
the answer of the difficult questions
the covers of the difficult book
the answers of the difficult question
the covers of the difficult books
the answers of the difficult questions
But what a complex syntax for the student!