This is the Cloudmages series by S L Farrell.
The bit with the seal and the magic fish is:
“Land-cousin, can’t you taste the salt in your blood? Thraisha is my name and Garrentha, who fought the dark-beast that attacked you, was of my milk.” The words came overlaid with the sound of the seal’s own language and came not from her ears but through Lámh Shábhála. Around Thraisha, there was a strange radiance in the cloch’s vision, something Jenna had never experienced before.
Jenna laughed in wonder, glancing back at Ennis with wide eyes. “Thraisha, you can understand me now when I speak?” Jenna asked, and she knew the answer immediately: her voice came back to her altered into the moans and calls of a seal.
“The language of Saimhóir is part of your blood, and Lámh Shábhála allows you to tap that part of yourself,” Thraisha responded. “And I have chased and swallowed Bra dán an Chumhacht, the first bright salmon of the mage-lights, which has come back to us. I am like you and I bear the marks. Aye, I understand you through Bradán an Chumhacht as you understand me through Lámh Shábhála.”
Jenna blinked. “You’ve eaten a fish that gave you the ability to tap the mage-lights?”
As you'll gather from that extract the books are heavily Celtic flavoured.
The master stone is found by the main character Jenna after a flash of light from the sky, and as you say, slow magic is mentioned many times in the books.
The trial you refer to is called the Scrúdú. Jenna takes the trial in the first book Holder of Lightning. She has to fight a beast called An Phionós who is indeed very much like a dragon. She fights well but is on the point of failing when:
The pressure was suddenly released. An Phionós dropped her, and Jenna gasped in pain and surprise as she fell back to the ground, struggling up to a sitting position with her legs folded underneath her. The beast coiled above her, the wings and body blocking the sky. “Why did you come here?” it raged at her. “I can take your life if you give it to me, but I can’t take a life that doesn’t come here willingly—She whose servant I am won’t allow that. Why would you do this?”
It glared at her, mouth gaping dangerously, then the eyes and its voice softened. “You don’t know, do you?” it asked.
Jenna shook her head. “I don’t understand. No.”
“Look,” An Phionos answered. “Look within yourself.”
An Phionós gestured, and Jenna saw herself as the creature saw her: a form of energy and light, her heart beating like a candle fluttering in the wind, and in her belly, a tiny flame burned.
“Mother-Creator . . .” Jenna breathed. She cupped her abdomen, as if she could warm her hands in that small radiance.
“Aye,” An Phionós answered. “You’re with child. You didn’t know?”
Jenna's granddaughter takes and passes the trial in the third book Heir of Stone.