The middle-distance runner - who races for Preston Harriers - made the trip to Estonia to pit his wits against some of the hottest prospects on the continent.
The 22-year-old former Pendle AC member failed to join international team-mates Joshua Lay and George Mills in the final at the Kadriorg Stadium in Tallinn.
But Tiarnan - who had beaten the duo at the Bedford International Athletic Stadium a couple of weeks earlier - showed encouraging signs in his first appearance at the European Under 23 Championships having missed out on qualification by 0.8 seconds.
After finishing his 1,500m heat in 3:46.31, which was almost six seconds slower than his winning time at the English Championships last month, he said: "It was tough!
"I didn't manage to qualify from my heat, but it was a good experience and a big learning curve for me. I'm proud of what I've done.
"I think the occasion got to me a little bit. I didn't run my own race or follow my own tactics. I just got a little bit lost in the moment, but that can only be expected.
"It was just a confidence issue. I've competed against the best in Britain but it was another step up to take on the best in Europe. A lot of the athletes have been there and done it before. It was a completely new experience for me so it will take time to adjust.
"In the immediate aftermath I felt as though I'd let myself down but I needed some time to properly reflect on it. I'd just run for Team GB in the European Championships and not many people can say they've done that.
"Team GB endurance coach, Helen Clitheroe, spoke to me after the event and told me that her first championships didn't go to plan. She told me to learn from the experience and not get too down about it."
The history student at the University of Leeds, who lives in Burnley, didn't waste any time when landing back on these shores.
After 16 hours of travel, the ambitious racer jumped straight back into action to break the four-minute mile barrier at the Diamond League meet in Gateshead.
"I managed to break the four-minute marker for a mile in Gateshead," he said. "That was a great achievement and showed me that the race in Estonia was an anomaly. It felt good to bounce back straight away.
"I was quite shocked because I was really tired from travelling, but I think the adrenaline kicked in. It was a big race on the BBC and I didn't want to let that opportunity pass me by."
The former Blessed Trinity RC High School pupil, coached by Andy Bibby, would love nothing more than to do it all over again.
However, he's prepared to play the long-game. "I don't want that to be a one-off. Now that I've qualified once I want to ensure that I become an established athlete for Great Britain
"I want to go forward and carry on competing at major championships. I didn't perform terribly in Estonia, I was just beaten on the day by better athletes. They've had the experience and that's now the pathway for me.
"The 2023 World Athletics Championships [in Budapest, Hungary] might be a good one to aim for. I should be starting to hit my peak at around that time. For now, with the end-of-season approaching, I'll just try to end the campaign with another PB."