The Fall of the Sea People

Arcadia: if you look closely, you can see the sheep

The Fall of the Sea People is an epic fantasy. The first volume, Venus and the Sea People, sees a singer return from life at The City to her home village nearly nine hundred years after she departs. She is befriended by two local girls, who persuade her to take them to the City. The rest of the first volume follows the rise to power of one of those girls, and ends on a cliffhanger.

Tropes used in The Fall of the Sea People include:

All Deaths Final: because raising the dead is forbidden art
Always Identical Twins: Geana and Eala. Possibly plot-significant in that they both carry a magical gene
Angsty Surviving Twin: The Mentor
Anyone Can Die: half of Eala's novice companions are dead before the end of the first book.
Arcadia: Anleacán. Éirime is even called The Shepherdess because she comes from Anleacán.
Arc Words: "Immortality doesn't mean living forever" and "The Sea People do not marry"
Atlantis
Casual Kink: quite a few
Charm Person: Initiation
Cliff Hanger: more literal than most
Constructed World: An in-universe example - Anleacán, possibly others
Cool Sword: the weapon of choice of the Sea People. Just having them made of metal makes them advanced weapons technology in the setting, but they also have battlefield communications and are used to direct magical attacks.
Country Mouse: Eala
Crapsack World: the City. And a lot of the rest of the world, too.
Deadly Decadent Court: the favourites, and their circles in turn
Distant Prologue: although it is all set in the time of Atlantis, the first chapter is set about 900 years before the rest. Somewhat averted in that the main character of the prologue is also a significant character in the rest of the first book.
Don't Go in The Woods: except that most Novices have to. Trying to avoid Forest Service is a major motivator for Novices.
Don't Split Us Up: Eala goes to Selection because she wants to stay with her sister. She was Selected, her sister was sent home. So if she hadn't gone, they'd have been together back home.
Eldritch Abominations: in the Forest. And Fómhar, helping on the waterfront in the City.
Everybody Has Lots of Sex: certainly some is lampshaded by Abhainne as a way to win favour from higher Ranks, more is possibly justified among the Novices as "Glad to Be Alive" Sex.
Fantastic Rank System: The City has three tiers of ranks, Novice, Veteran and Immortal, with eight levels in each. The Fantastic Honorifics are simple: someone in a higher tier is addressed as "Lord/Lady", and the Mentor is addressed as "Mentor". But peasants address all of them, including the Mentor, as "Lord/Lady".
Flat Earth Atheist: Literal examples, especially justified by Aclaí, who can stand in Anleacán and worry who created the world, since he did it himself.
Foregone Conclusion: If you recognise the epigraph, you know the subject is Atlantis
Free-Love Future: well, free love past
Functional Magic: of course
Green Eyed Red Head: redheaded twins, no less!
Hair of Gold: mostly played straight with Eala, subverted with Geana
Healing Factor: Immortal Breath is a skill which must be learned, and kept up.
Humans Are White: Averted. Many of the characters in the City are recognisably of other races, and several are described as mixed-race. It appears from the description of the Favourites that the Mentor prefers other races to the dominant Celtic type. Justified in Anleacán, which was designed and created by a white teenager.
Immodest Orgasm: Éise when the Mentor gives her boyfriend back
Irish Names: or at least names that have a meaning in Irish. A glossary and pronunciation guide is provided.
Lady Drunk: Éirime is a rather older version than most.
Land of One City: one city that counts, anyway
Language of Magic: the Sea People utter Words of Power; in Caora's case, she sings them
Life Will Kill You: "Immortality doesn't mean living forever"
Loads and Loads of Characters: Sixty named characters introduced in the first volume
Magic Knight: no Squishy Wizards these!
Magitek: much of it weapons-grade. And they need it.
Matchmaker Crush: Rónmór is a particularly grim example
Mayfly-December Romance: Geana and Éirime, and Eala and the Mentor if she doesn't figure out Immortal Breath in time
Mildly Military: there are ranks, and a chain of command, but it is the effects of Initiation which keep the City focused.
Mugging the Monster: some hunters attempt to rob the Mentor.
No Periods, Period: Justified when Eala enters the City, as a magical effect
No Social Skills: Cathúa
Not-So-Imaginary Friend: the Mentor and his dead brother
Our Fairies Are Different: the Forest People are genetically engineered to guard humanity from the Forest
Outgrown Such Silly Superstitions: poor Abhainne and her household gods
Portal Door: One way to get to alternative universes. The Gate that leads to Anleacán is near the City, but there is another one, the Sky Gate, on the other side of the world.
Punctuation Shaker: averted as the accent marks (called fada in Irish) come from a real-world language
Pygmalion Plot: Literal example, with Aclaí as Pygmalion and Éirime as Galatia - with Anleacán for her pedestal. Then she is Selected and he never speaks to her again.
Really Seven Hundred Years Old: Damhánalla, Éirime, probably others
Sacrificial Lamb: flocks of them. Three quarters of Novices die within a year of being Selected.
Sacrificial Lion: starting with Rósa, Lách and Learó and possibly ending with Eala
Spirit Advisor: played straight with Rónmór; horribly subverted by the Mentor's dead brother
Stranger in a Familiar Land: Éirime is this
The Fog of Ages: the Mentor can't remember what his wife looked like.
The Older Immortal: It is the older Immortals who run the City
Twin Desynch: inevitable after Eala is Selected
Twin Switch: Geana and Eala did this to Ordóg
Two Siblings in One: strongly implied with the Mentor and his dead brother
Unobtainium: The Metal the City uses for swords and armour. Presumably Orichalcum
Unresolved Sexual Tension: Aclaí and Éirime
World Building: Anleacán, in-character and lampshaded by Cathúa over dinner with the Governor of the Two Rivers