Stensness hit the scoreboard for Viking as they gave German giants Bayern Munich a huge fright in a dramatic high-scoring draw in Norway, and Irankunda was influential and provided an assist in Sporting Lisbon's win over Lens in France.
In other results, Barcelona survived a scare against Galatasaray, Arsenal rolled over Lille, and there was more misery for Manchester United.
Barcelona's 100 per cent start to the season was under threat down 1-0 to Galatasaray in Istanbul, but the Spanish champions rallied to a 4-1 win.
Bayern were in even more trouble against Viking in Stavanger, trailing 2-0 at halftime.
Viking opened the scoring in just the second minute courtesy of a cracking header from Australian Stensness, who has played two games for the Socceroos.
Peter Christiansen heaped more pressure on the visitors when he added a second in the 38th minute.
However, England striker Harry Kane turned the game around with two goals for Bayern after the break.
Luis Diaz looked to have completed the comeback, but a red card for Michael Olise, and a 90th-minute goal from Herman Haugen provided another twist.
Olise's bizarre sending-off paved the way for the Norwegian team to rescue a draw in their first Champions League home game since 1992.
Shortly after Bayern went ahead 3-2, Olise picked up two yellow cards in quick succession - the second for tossing the ball at opponent Zlatko Tripic while the referee was still booking him for his first foul.
Tripic then provided the cross that allowed Haugen to head in the equaliser in the 90th minute.
In France, Sporting Lisbon sealed a 2-0 win at Lens through goals from Luis Suarez and Issa Doumbia.
Star Socceroo Irankunda stamped his authority on the match for Sporting, providing an assist for Suarez's 22nd-minute opener during his influential 73 minutes on the field.
Elsewhere, Arsenal cruised to a 3-0 win at home over Lille and Atletico Madrid piled more pressure on United coach Michael Carrick with a 2-0 win at the Metropolitano.
Inter Milan fell to a surprise 4-3 home loss to Club Brugge, and Villarreal beat Napoli 3-1.
It's 10 straight wins for Barcelona after they maintained their perfect record in all competitions despite early trouble in Turkey.
Aleksey Batrakov gave Galatasaray the lead before his team had a second goal ruled out for a tight offside.
Joao Cancelo's goal levelled the scores just before halftime and it was all Barcelona after that with goals from Fermin Lopez, Anthony Gordon and Karim Adeyemi.
Defender Lisandro Martinez hit out at Manchester United's hierachy after the club extended their winless run to five games.
Carrick's team have won just two games in all competitions this term and fell to a fourth loss after goals from Pablo Barrios in the first minute and Ademola Lookman in the second half.
"I have to control myself a lot, but sometimes it's a reflection in the mirror of how is our club at the moment," Martinez said. "We did a fantastic season last season. And now we expect more from the top, to bring more players to compete because many teams bring many players as well."
In London, Bukayo Saka stepped off the bench to score a goal in Arsenal's 3-0 win over Lille.
Jurrien Timber had given Arsenal the lead in the 64th minute. Saka came on after 65 minutes and within two minutes set up at goal for Eberechi Eze. Four minutes later he was on the score sheet himself.
Inter were on the losing end of a seven-goal thriller against Brugge, who pulled off an upset in Milan.
Nicolo Tresoldi is emerging as one of the brightest prospects in Europe and enhanced his reputation with a hat-trick for the Belgian visitors, while Lautaro Martinez scored twice for Inter.
Azerbaijani outsiders Sabah claimed their first win in the league phase of the competition by beating Slavia Prague 2-0 in Baku.
Brajan Gruda's first Champions League goal lifted Leipzig to a 1-0 win over PSV Eindhoven
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