The manager described how the goat ran amok outside, eating flower baskets and frightening staff and customers as they tried to get into the building.
"It was running around the place, chasing customers and jumping up on cars," Mr Smyth added.
"It also butted one of our regular customers, Billy, who comes early every morning to get his paper. We had to quickly open the doors and drag Billy inside.
"Thankfully the only real damage was to the plants which we keep outside. It ate them all, but I thought I am not going out there to tell it off."
The pensioner was not hurt. The unusual visitor appeared at the Eurospar doors at about 6.30am.
Footage of the animal at the shop window has attracted thousands of views online.
One customer tried to use the ATM, but ran back to her car when she spotted the goat.
Mr Smyth said: "We were the only light on in the area so that's probably why it came over. There was not much we could do to stop it. The goat just kept running up to the window and butting it.
"Any time there was movement somewhere else it would run towards that. One of our staff had to jump into a customer's car because it came running towards her."
The goat was eventually stopped when a member of the public, believed to be the animal's owner, grabbed it by the horns and took it away.
Mr Smyth said: "It was an interesting morning."
