WHAT is the population of Paisley?

About 75,000? Give it some time and that is how many will claim they were at Hampden to witness this little piece of history. Only a knot of around 5000 St Mirren fans actually had enough courage and faith in their team to turn up and see what would happen against Celtic. What a reward they got for that commitment. St Mirren pulled off what many previously considered unthinkable, and remarkably brought down Celtic in the Scottish Communities League Cup semi-final.

It was such a wonderful day for the Paisley club that the side needs to finish the job by winning the final against Hearts on March 17. They will be up against a weaker team that day but one that will surely rise to the occasion in a way that Celtic abjectly failed to yesterday. Too many of their players went through the motions as if the League Cup was beneath them. The collective mediocrity of the performance was an affront to the 20,000 or so of their own supporters who came along. St Mirren played as well as they could have hoped to, but Celtic must consider going out of the cup, with such a tame defeat, as unpardonable.

St Mirren are 11th out of 12 in the Clydesdale Bank Premier League for a reason. They have been unimpressive for most of the season and gave no inkling of the tenacity and energy they would bring to this scrappy but compelling semi-final tie.

In Esmael Goncalves, Paul McGowan and Steven Thompson they had three goalscorers who ruthlessly capitalised on the carelessness and mistakes in Celtic's play. Goalkeeper Craig Samson saved a penalty and John McGinn and Conor Newton were tireless in midfield. St Mirren punched above their weight all over the pitch.

Hampden has wounded St Mirren before. Three years ago the final was theirs for the taking against a nine-man Rangers team only for them to fall to a Kenny Miller winner. No wonder they were biting their fingernails even after Thompson's goal put them 3-1 up on Celtic with 21 minutes left, lest they throw it all away again. They were on stronger ground this time – Charlie Mulgrew scored Celtic's second with literally the last kick of the match – and up against a team that, for once, showed a woeful lack of character, belief or quality.

What a wretched day it was for Celtic. A possible treble was cheaply thrown away with the worsening of a poor recent record at Hampden. That is now three straight defeats here and only four wins from the last nine visits. If a cup final defeat by Rangers might have been expected in that sequence, then the losses to Ross County, Hearts, Kilmarnock and now St Mirren were not.

The postmortem will be long and merciless. It remains a curious characteristic of Neil Lennon's Celtic that no matter the great heights they scale – beating Barcelona et al – they periodically stumble when they are the overwhelming favourites. Hampden was cold and dreich, with swathes of empty seats doing little for the spectacle, but the great sides – sides which win trebles – regularly overcome far greater discouragements.

St Mirren rose more to the occasion than Celtic. They capitalised when they were afforded too much room in midfield. When Celtic were slow into tackles, St Mirren won them and tried to build moves. Their lack of quality often let them down, but they always attempted to play and they made this their day when the expectation was that they would be buried.

Goncalves, the 21-year-old Portuguese newly arrived on loan, stole in front of Mulgrew to poke a finish into the corner for the opening goal in eight minutes. Gary Hooper, whose passable performance elevated him above any team-mate, clipped the bar with a shot and then Georgios Samaras did so with a header. St Mirren held on until Lassad Nouioui and Scott Brown opened them up down the left, the latter firing a ball into the goalmouth which Hooper tapped in.

When Jim Goodwin was harshly judged to have handled – Lassad's shot flashed off his chest on to his arm – Celtic had a penalty and the game seemed to have its turning point. Instead Mulgrew's limp kick was saved by Samson, continuing Celtic's appalling ratio of converted penalties.

Mulgrew had an awful day regardless of his late, long-range goal. When he threw up his arms at Gary Teale's cross it was a handball and the game had its second penalty, this one buried firmly to Lukasz Zaluska's left by McGowan.

The tie had taken an unexpected turn and when Thompson expertly volleyed a Marc McAusland cross past the goalkeeper – Celtic's defenders were nowhere – Hampden suddenly realised something exceptional was unfolding.

Mulgrew, Kris Commons and Joe Ledley, twice, had scrambled and desperate attempts but St Mirren were never subjected to a prolonged battery. Celtic finished the game as they had performed throughout: making the odd chance but never raising above a numbing mediocrity.

As the Celtic support began to melt from the terraces the St Mirren section went wild. Many more will mobilise for the cup final but, boy, this semi-final fed the 5000.