These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
In "The Wasting Sickness of Cu Chulainn." Cu Chulainn spends a year bedridden from nightmares about two women whipping him nonstop. When Emer finds out where he's been all this time she heads over to Emain Macha, shakes him out of it, and berates the Kings of Ireland for not having the balls to help, all the while complaining that she has spent a full year without her husband's company.
When Emer finds out that Cuchulainn is sleeping with some random sidhe girl called Fand, she marches over with a knife to cut a bitch. What does Fand, this supposedly superior-in-all-ways woman, do? Stand there and cry. Meanwhile, Emer starts challenging Cuchulainn to a fight—if he wins he can leave her for Fand, and if SHE wins he stays with Emer. Does Emer care that Cuchulainn outclasses her and doesn't want to put his own wife in horrible danger? No. In fact, she calls him a pussy for being afraid to answer a woman's challenge.
Ho Yay: Cuchulainn with Ferdia, his childhood friend whom he meets and fights with again in adulthood. Not only do they kiss, they do it three times.
May be a severe case of Misplaced trope, kissing, especially multiple times on the cheeks or even briefly on the lips is a common form of greeting valued friends acquaintances and relatives right up to the modern day in Europe without any kind of sexual connotations. Especially given the Rest of Cu Chulainn's sexual proclivities (Because bisexuality doesn't exist)note imbecile.
However, any traces of homosexual behaviour were eliminated from known Celtic records after conversion to Christianity, so it could also be a relic of the pre-Christian version.
Considering the story involved Cuchulainn "piercing Ferdia's anus", it seems to be the case of a barely disguised myth.
And there ARE a lot of LGBT-oriented rituals that explicitly invoke Cuchulainn and Ferdiad. Whether they work or not, the figures in question certainly don't mind.
Jerkass Woobie: Aoife. Finding out the dude who knocked you up has been married to another woman for years is horrible, but that doesn't excuse sending your prepubescent son to his death.
Memetic Badass: Cu Chulainn has shown up on Badass of the Week, and his penchant for ridiculous quantities of bloodshed has become a Running Gag on Bettermyths.com (which also claims that he was immune to the Pains of Ulster because he ripped them out of his stomach and beat the crap out of them until they turned into guns. And then he shot the pope. Just 'cause).
Cuchulainn has FOURTEEN FINGERS AND FOURTEEN TOES
ALSO FOURTEEN PUPILS
WHY FOURTEEN PUPILS?
BECAUSE TWO PUPILS IS NOT ENOUGH TO WITNESS THE SHEER QUANTITY OF HOMICIDE HE COMMITS
Magnificent Bastard: Lugh, thought by some myth-tracers to share a concept-God with Loki of Norse Mythology. The key difference here being that the Celts didn't have anything against tricksters and so Lugh spends his life a badass rather than being tortured.
Medb/Maeve of Connaught, who raised an army and invaded Ulster just so that she could steal a magic bull to match the one her husband owned. Some say the story is about the primacy of female or male lines of inheritance. Others say it's about Medb and Ailill being irresponsible jerks. (For certain values of "jerk.")
Nightmare Fuel: The Irish hero Cú Chullain is a scary guy at the best of times, but his rístrad (a battle madness taking the form of some sort of convulsion brought about in moments of extreme indignation) is another matter (see this description). Anyone watching this would no doubt have so pretty nasty nightmares - providing Cú Chullain doesn't kill them before they have a chance to get to sleep again.
The murder of the hundred Irish warriors in the tale of Branwen is pretty horrifying. They hide in sacks of flour and wait to ambush the British. Efnisien, a British chieftain, grips each bag individually and squeezes until he can feel his fingers sinking through their skulls into their brains.
The Irish warriors were sent to kill Efnisien because he'd gotten so angry about how Branwen got married to Matholwych without his consent that he cut the ears and tails off of Matholwych's horses. Considering this was set in medieval times, when horses were even more of a status symbol than they are now, that doesn't bode well for the story.
Tear Jerker: The Children of Lir. As children, they were turned into swans for nine hundred years, and when their curse was finally broken, they were so old and withered that they died almost immediately. All that they could request was to be buried together (and, in some versions, to be baptized as Christians).
Cuchulainn, period. He accidentally killed his only son, accidentally pissed off the goddess of war just because he was too exhausted after a battle to bother with another strenuous activity, and died a slow and agonizing death. (Still died like a badass though. He tied himself to a tree as he was dying because he wanted to die standing up. No-one went near his corpse for ages because they were scared shitless he might still be alive. In the end they only decided it was safe because the birds had started eating him... and then his sword arm coincidentally fell and decapitated someone who tried to loot his corpse.)
The story of Enya, Aengus' foster-daughter in "The House of the Two Pails." A notably depressing take on Christian conversion.
Unfortunate Implications: There are a couple for Taliesin's past self in his origin story. Cerridwen was brewing the potion of Awen (Knowledge or Inspriration depending on the translator) for her son Morfran, who was so ugly he could never leave the house, with the intention being that people would look past that if he was smart. And then Gwion Bach steals it after all that effort (albeit accidentally) and goes on to not only become a folk hero with no repercussions but also get reincarnated as someone ungodly beautiful, while Cerridwen and her son are never mentioned in his story ever again.
It wings all the way around to a double example when Morfran's only mentioned 'ugly' feature was his jet black skin. Which is probably coincidental, given how unlikely it is anyone involved had ever so much as seen an African, but still.
What an Idiot: For someone whose epithet is "the Wily," Emer's father King Forgall isn't particularly bright. He disapproved of Cuchulainn for some unmentioned reason, so he tried to marry Emer to someone else. Repeat: He tried to force The Chosen One's intended bride to marry someone else. Luckily, the other "suitors" were too terrified of Cuchulainn's potential retribution to accept. On the other hand, he still locked her up and started guarding against Cuchulainn's return, which resulted in Cu Chullain going on a Roaring Rampage of Revenge to save/elope with Emer.
King Pwyll of Welsh mythology. The poor guy catches one Idiot Ball after the other.