I've lived outside Ireland for well over 20 years but have been stuck in the country due to the pandemic. During the past few months, Irish has been on my mind for the first time since I left school, and I've been entertaining myself revising things I'd forgotten or never knew in the first place because, like most people, my Irish was never very good.
Looking at the information that's available online, every pronunciation guide makes a huge deal out of the slender/broad pairing for consonants, stuffing fiddly superscripts for velarised and palatalised sounds into their IPA transcriptions of words, and I began to get pretty baffled about this issue the more I read about it. I learned Munster Irish as a child and, to me, the addition of an i/e does precious little to most adjacent consonants but does a hell of a lot to adjacent vowels.
A poster above mentioned sloinn/slonn. To me, the "n" at the end is the same in both - just a plain "n", no glide. The difference is in the vowels: the pronunciation of the two words approximates to "slin" vs "slun".
To turn to your examples:
"mar" vs "mear" - "mar" sounds pretty much like the EN verb "to mar", while "mear" has a sound much closer to the "a" in "apple" (as far as I can describe it, anyhow). The vowel sounds are by far the major source of distinction - personally, I wouldn't insert a big fat -y- sound after the "m" in "mear".
"fara" vs "fear" - again, the first "a" in "fara" sounds like the "a" in "to mar" and the "ea" in "fear" sounds quite like the "a" in "apple". "Fear" doesn't sound like "fʲæɾˠ", the IPA rendition in Wiktionary - at least, the "f" doesn't.
"ansin" vs "sean" - the "n" at the end of both words is the same. It's just an n, nothing like the Spanish n with a tilde.
EIf you haven't been able to hear the supposed differences, it's not surprising - they don't exist the way the literature suggests.
The online dictionary www.focloir.ie includes pronunciations in the 3 major dialects, and I've been surpised at how closely my intuitions about pronunciation match the Munster standard despite the fact that my schooldays are now so far in the past.
I can't for the life of me make out where the online literature's exaggerated treatment of slender/broad consonants came from. There are some clearly distinguished pairings like s, d and t. While slender/broad "s" is conditioned by the adjacent vowel, this vowel does more than just change the consonant - "san" sounds like EN "sun" but "sean" sounds like "shan", not "shun". Again, this fact seems to be ignored by online pronunciation rulebooks, which would have you believe that the only distinction is "s" vs "sh". My own pronunciation instincts sometimes have me add -y- at times though I have no textbook rationale for it (look up "philosophy" in focloir.ie and contrast Munster with the other two dialects), but I've begun to suspect that the rules surrounding pronunciation are far more complex than most books let on.