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The Irish striker scored all three goals in Southern's 3-1 win over Hawke's Bay United in Napier on Saturday, leaving it with 24 league points.
That was confirmed to be enough for fifth yesterday, when Waitakere United beat Tasman United 5-3.
It left Southern thrilled and coach Paul O'Reilly said it was great progress.
He was happy with the team's performance and spoke especially highly of Coughlan.
"He was phenomenal.
"Of all things, for an Irish player to score a hat-trick on St Patrick's's Day, he was outstanding.
"It was the perfect hat-trick, left foot, right foot and a header.
"He could have had another two or three, he hit the crossbar and had two or three chances that were saved.
"He was just head and shoulders above everyone else on the park. He was outstanding.''
Coughlan scored his first in the 23rd minute, getting on the end of a long ball and putting it through goal keeper Ruben Parker's legs.
He made it two just before halftime, scoring on a penalty after being dragged down in the box by Jorge Akers.
A third came in the 62nd minute, as he found the back of the net with a header.
Hawke's Bay responded late via Sam De St Croix, but the deficit was too much for the host to overcome.
The Irish influence in the team was not just Coughlan's, as O'Reilly and several of the others in the team hail from that country.
While St Patrick's Day had not been a focus, he said it could have brought extra motivation.
"It wasn't something we focused on, I'd have to say.
"But obviously, we're a long way from Ireland and St Patrick's Day is a massive festival around the world.
"So I'm sure the lads were probably having a think about it, sometimes you miss home.
"I'd imagine Garbhan was feeling a bit of Irish pride and a bit of nostalgia with the day.
"It might have given us an extra percent or two, maybe.''
He said he felt the team deserved its placing on the table and was thrilled to be "the best of the rest''.
Southern finished with six wins, six draws and six losses.
However, he acknowledged the top four remained a long way ahead of the rest of the league.
In next weekend's semifinals, Auckland City hosts Eastern Suburbs in an Auckland derby, while Canterbury United travels north to play Team Wellington.
Southern United 3
Coughlan 23, 44, 62
Hawke’s Bay United 1
De St Croix 83
Halftime: 2-0 Southern