Declan O'Brien performance of "When a Pint is Poured," a hilarious Guinness-inspired cover of Johnny Mathis' Christmas classic "When a Child is Born," is going viral on social media.

O'Brien, a comedian and entertainer, performed his rendition of the song in Briody's Pub on Dublin's Marlborough Street on Saturday night, December 9.

O'Brien wrote "When a Pint is Poured" more than 30 years ago and has regularly performed the song at Christmas functions over the past three decades but said he was "shocked" to see the song go viral after his most recent performance. 

A video of the performance has been viewed more than 152k times since it was uploaded on social media on Sunday night. 

"Am I surprised? I'm fecking shocked," O'Brien told IrishCentral. "I've had messages from Boston, San Francisco, and Australia." 

The song substitutes Mathis' original lyrics about the birth of Jesus Christ for the agonizing wait for a pint of Guinness to settle. 

"The barman pours it halfway up the glass

He puts it down and he lets some minutes pass

But then he goes away, you think you've been ignored

But he comes back, then your pint is poured." 

The festive performance was shared on TikTok, where it's racked up nearly 65k views, as well as on Facebook, where it's drawn more than 96k views.

@londonrazor ♬ original sound - LondonRazor

O'Brien, who has been a regular contributor to RTÉ Liveline's Funny Friday and the Gerry Ryan Show over the last 30 years, said he was inspired to write the song after realizing that the original song's structure lent itself to the process of pouring a pint of Guinness. 

"There's a part in the original song when Mathis begins talking instead of singing and it starts with the line 'all this time you've been waiting'. That pretty much matches the experience of waiting for a pint of Guinness to settle," O'Brien told IrishCentral. 

O'Brien performed the latest rendition of the song on Saturday night while at a Christmas party with Parnassus, a drama group he helped set up in 1981. The group regularly rehearses at Briody's and O'Brien was invited behind the bar to perform the song. 

A taped version of Parnassus' 1995 comedy show "World Cup Me Arse," which O'Brien co-wrote, went Gold after reaching number four in the Irish album charts in April 1995. 

The show ran for a year, including a stint in the Olympia Theatre, and centered around the 1994 FIFA World Cup in the USA.