The querent has confirmed that it was "Death Eater takes a Holiday" by Lee Lee Potter.
Albus spells a few Order members back to their seventh year age, and sends them to spend time at Hogwarts for a holiday. The Headmaster’s intent is for them to learn that Harry is indeed mature for his age, so that he will be accepted as a full member. What will happen when Severus returns from his time as a teen, to find he has spent the better part of the week entwined with a certain Gryffindor? And, how do their lives change afterwards?
Kieran first appears in chapter 33, I believe.
Severus nodded and stepped into the fire. He had so much to think about he decided not to think at all just yet. He really needed to get out of his bloody robe. As he stepped into his bedroom he immediately felt the cool air. Spelling the room warmer again, Sev remembered Albus' request. He lit a fire and threw in a pinch of powder from a small box on the mantel. "Kieran Donnelly," Sev called into the fire. A moment later the head of a man with wiry brown hair and a three-day-old beard appeared.
It does involve a relationship between Harry and Snape, although the only mention I see of a potions conference or convention is in a story that Snape tells about his father, the Dark Wizard:
"Good memory," Severus commended him. "He was not a nice wizard, but at some point he became aware that I was to replace him. He had angered the Dark Lord and was going to be killed as soon as my training was complete. Slowing down my training would be obvious, so instead, he taught me everything he knew, every single detail and nuance to potion brewing known only to Masters. Then, to continue my education, he took me to symposiums, conferences, and helped get me connected with ingredient suppliers all over the world. He always had a full agenda to show our Master that he still had things to teach me."
I'm pretty sure I found it with a search for
hpfandom.com snape kieran. I had discounted it as an answer because I figured the de-aging part would have been pretty memorable, and because I could find no mention of the conference.