Both directors have the ability to tell solid stories, but I think it's telling that even some of Miyazaki's early works aren't held as high as a lot of his later work, but even a "not as good" Miyazaki movie is still pretty good. The same seems to hold true with Tomm Moore, who tells a story that isn't quite on part with stories like Spirited Away or Frozen but stands close behind them by the things it brings to the animated film "party" that other sources don't.
Song of the Sea is the story of Ben, a young boy whose mother disappears one stormy night, leaving behind a new baby sister Saoirse (pronounced "Ser-sha"). His father, Conor (resident lighthouse keeper and giant of a man) is emotionally destroyed by her disappearance, and even six years later doesn't really seem able to raise either child the way he should. Ben's protests at needing to constantly watch after his sister fall on deaf ears, and his only real friend seems to be his pet dog Cú. Saoirse, meanwhile, is yet to speak a single word or make a sound.
After an incident involving Saoirse, a coat that Conor left locked in a trunk in his closet, and the water around the island, Conor's mother decides it's best to take the two children and raise them herself. Upon arrival to the big city, Ben almost immediately decides the best course of action is to find his own way back home again, but finds Saoirse tagging along with him. It isn't soon after that Ben learns that his sister is a selkie, a creature of legend whose "song" will "free the fairy creatures" and save them from Macha, an elderly fairy woman who captures emotions in jars and turns her victims to stone. However, Saoirse is also growing more and more ill and their travel continues, and needs to get back to the lighthouse before it's too late.
Ben's relationship with Saoirse felt extremely organic and real. Speaking as an older brother myself, I know there were many times I felt both extremely protective of my sister while also wishing she could go away without needing someone to follow up on her. Fortunately, my parents were better than Ben's were (neither one just up and vanished from an island one day), so it wasn't my responsibility to raise her. During the moments where Ben feels frustration towards his sister, I nodded slightly in sympathy for him, but I also noticed that whenever Ben was presented the opportunity to not need to deal with his sister any more, he always chose to keep her close and protect her.
The story is packed full of myth and legend, with story points touching on everything from the creations of the oceans to smaller, personal stories. Through the whole thing, Ben is there, prepared to face danger to protect his sister and save these strange beings he used to hear about in stories from his mother (it's funny that Ben actually knows more about the creatures than they themselves do, filling in holes from their memories with the stories and songs his mother told him, even if he never knew they were all real).
The artwork and animation are simply gorgeous and worth the price of watching alone. A mixture of CG and hand-drawn artwork (sources I've read indicate it's more hand-drawn than computer-generated, and I'm inclined to believe it), the settings don't quite reach the same fantastic touches as, say, Spirited Away but are done in such a distinctive and unique style that you find yourself marveling at the little background touches anyway.
It's not a perfect movie. There are a few beats in the film where it falters, characters get a bit flat, or the story seems to pause for a moment before regaining its focus. There are a few times where I marveled at a character's ability to be extremely clueless until the plot demanded they suddenly realize what was "wrong" around them, and one character makes a rather astounding leg of the journey on their own, but makes their presence known in such a charming way I quickly forgave it.
Honestly, with this much life taken from old myths and legends that aren't your standard Japanese, Roman, or Greek fare, it makes me anxious to wait for more coming from Tomm Moore, as legends and stories from the Isles are something that just seem to keep almost making it into the mainstream but keep falling shy in favor of another story about Hercules or moon princesses (Sailor Moon or Princess Kaguya, take your pick).