The show's cast and creators managed to keep a lid on the veteran actor's cameo, that remained a secret up until the moment season three aired on Tuesday night.
Show creator Lisa McGee revealed how the cast and crew kept the secret under wraps.
"Thanks for watching lads! Our code name for Liam during the shoot was ‘the big fella,’ arguably quite an easily cracked code," she tweeted.
"What an honour to have him in our wee show. Our very own Northern Star."
Fans were delighted with Taken star Neeson's appearance as a police officer tasked with interrogating the girls - and the fella - after a break-in at their school.
At first they thought the mystery copper was Line of Duty's Adrian Dunbar as Neeson emerged from the shadows.
Despite Neeson lending a superstar quality to the first episode, many fans wrote that it was Uncle Colm, (played by Kevin McAleer), who stole the show."Uncle Colm boring *actual Liam Neeson* to death is everything I never knew I needed - god I have missed this show," wrote Twitter user Erin.
"Satellites passing over Ireland at 21:33 GMT picked up a collective gasp in space when Liam Neeson popped up in Derry Girls," wrote Belfast comedian Paddy Raff.
"Keeping Liam Neeson's guest appearance under wraps for this must have been the best kept secret in Derry," Twitter user Caolán Mc Aree wrote.
The Channel 4 comedy aired the first episode of what Ms McGee has confirmed is set to be its third and final season, on Tuesday night.
The show will air on C4 every Tuesday at 9.15pm, and each new episode will be available on All 4 after the show airs.
Derry Girls seasons 1 is still available to be viewed on Netflix in the UK and Ireland.
Season 2 is no longer available on the streaming giant's services.
Netflix explained last July: "We'll let you know when it's coming back as soon as we can. In the meantime, it's available now on All 4."
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The show's third season is not yet available to watch on Netflix.
The series stars Saoirse-Monica Jackson as Erin, Louisa Harland as Orla, Nicola Coughlan as Clare, Jamie-Lee O'Donnell as Michelle, and Dylan Llewellyn as James.
The critically acclaimed cast also includes Tommy Tiernan, Ian McElhinney and Siobhán McSweeney.
Filming on the show's third season had been delayed due to the pandemic.
Ms McGee, who is herself from Derry, has previously described writing the show as a "love letter" to her home city.