The Ireland AM host and the panto producer, who had a lavish civil ceremony at the Mansion House in 2011, opted for a more low-key day with just Alan’s brother Kevin and Karl’s sister Ann present as witnesses.
They chose September 30 as the anniversary of their civil union and of the date they met.
"23 years ago today we met, 15 years later we had our civil ceremony and because the Irish people said 'YES', we can say 'I Do'. I couldn't be prouder marrying this gorgeous man," Alan said, via a statement issued after the service.
"Everyone who meant anything to us attended our civil ceremony and we just wanted to do this for us," Karl added.
"Keeping it secret was the hardest part. I'm so happy to marry the guy I love and respect so much, now we can live happily ever after."
Alan famously popped the question to his long-term love live on air last year.
At the time, he defended his decision to propose so publicly, saying: "We can say, we are legally married in Ireland. It has taken so many years," Alan said on the Anton Savage Show on Today FM.
"The year we met, it was still illegal to be gay in Ireland. We could have been imprisoned as two consenting men making love in their own house.
"I can’t see why anybody wouldn’t want that on a piece of paper – to see that they’re legally married on a piece of paper – not civil partnered."