Mr Gallagher, aged 42, was popular and well-known as someone who worked at a local pub and in retail over the years.
One friend posted on social media to say that he was “devastated” by the loss of his friend and described him as a “legend”.
Another local described Mr Gallagher as a “true gentleman”.
Many others expressed their condolences and said they were thinking of Mr Gallagher’s family at this time.
Local councillor Grace Uí Niallais has offered condolences to the family of the deceased.
“The community is shocked and saddened to learn that a man has tragically died after a road collision on the Strand Road in the early hours of Sunday morning,” she said.
“My immediate thoughts are with this man’s family and friends at this very sad and difficult time.
“Police have appealed for anyone who may have information or video footage that can assist their enquiries to bring it forward.”
In a statement, the PSNI confirmed Mr Gallagher had died following the road traffic collision.
Detective Sergeant Arkins of the Collision Investigation Unit said: “Colleagues from other emergency services attended, but sadly Mr Gallagher passed away in hospital.
“Our enquiries are ongoing, and we are appealing to anyone who was travelling in the area at the time of the collision, or who has any CCTV, dash cam or mobile phone footage that could assist with our investigation, to contact us on 101 quoting reference number 185 of 11/02/24.”
Police had appealed for information following the collision in the early hours of Sunday.The road remained closed throughout Sunday morning as the police carried out investigations.