Today mum Mary (51) said "things just aren't adding up" after her son didn't return home.
The last time the catering worker was seen was at around 3am on Christmas Day at the cottage he rents with housemates on Sandy Lane in the seaside town of Rush.
He sent a text to his girlfriend at 10.30am and that was his last known contact with anyone.
Since then, his mother and siblings Stephen (29) and Suzanne (27), have been searching for him, putting up posters in Rush and Lusk.
Garda divers yesterday searched a large water tank close to the cottage.
A land search was stepped up in the north county area at the weekend while the coast from Balbriggan to Rush was scoured.
Paul Byrne is 5ft 5in tall with blue eyes, a thin build and short light brown hair.
Anyone with information on his whereabouts is urged to contact gardai.