The Irish league runners-up pulled off a stunning 1-0 victory in Bulgaria last week to make them favourites to progress to the next stage of the competition.
Match preview
Similarly to how they progressed against NS Mura - who beat Tottenham Hotspur in this competition last season - in the previous round, St Patrick's displayed a masterclass of defensive organisation and application in Sofia last week.
Tim Clancy's side kept a clean sheet in Slovenia to force penalties, and The Athletic were defying the odds once again in another eastern European country.
However, this time they went one better, with Serge Atakayi's 87th-minute winner sparking wild celebrations among his teammates and the coaching staff.
With a lead to protect at Richmond Park on Thursday, Clancy will hope there are more gaps for his side to try and exploit in transition, but the 38-year-old will be equally aware that his side's forthcoming opponents will throw everything they have at them.
Having thrashed Makedonija Gjorce Petrov and Beroe Stara Zagora by four goals apiece on home soil within the previous week, it is fair to say a home defeat to St Patrick's was not in the script for the Bulgarian runners-up.
However, CSKA Sofia could not break their stubborn opponents down, with five yellow cards for the visitors illustrating how hard they worked to keep the scoreline level.
At the very least, though, Sasa Ilic will have demanded his side avoid conceding at the Bulgarian Army Stadium, but that is exactly what they did with Atakayi's sucker-punch.
As such, they have a huge job on their hands to turn things around in Ireland on Thursday, but Ilic will be reminding his more talented players that an early goal can completely change the complexion of the tie.
St Patrick's Athletic Europa Conference League form:
St Patrick's Athletic form (all competitions):
CSKA Sofia Europa Conference League form:
CSKA Sofia form (all competitions):
Team News
St Patrick's do not appear to have any injury or suspension concerns facing them ahead of Thursday's return leg.
With their weekend match against Shelbourne being postponed, the home side have had a week to prepare for this clash, and as such it would be a surprise if Clancy makes any changes from last week's starting XI.
Atakayi will hope to make an impact from the bench once again, while Eoin Doyle will expect more chances to be created for him on home soil.
CSKA Sofia, meanwhile, will travel without Ivan Mitrev as the winger continues his rehabilitation from a fractured leg.
Like their opponents, The Army have also had a week's rest, but Ilic may be tempted to make changes after such a disappointing performance and result in the first leg.
Georgi Yomov could start after making an impact from the substitutes' bench, while Brayan Moreno could receive the nod up front.
St Patrick's Athletic possible starting lineup:
Anang; Brockbank, Redmond, Grivosti, Breslin; Lennon, O'Reilly; Cotter, Forrester, King; Doyle
CSKA Sofia possible starting lineup:
Busatto; Donchev, Mattheij, Petrov, De Nooijer; Youga, Shopov; Garcez, Yomov, Tufegdzic; Moreno
We say: St Patrick's Athletic 0-1 CSKA Sofia
We expect another tight encounter at Richmond Park on Thursday, with the visitors determined to rectify last week's surprise defeat in Bulgaria.
We would not be surprised to see another penalty shoot-out for St Patrick's in this competition, with the home crowd potentially pulling them through to the next round.