Hansen is due to undergo surgery after sustaining the injury during Connacht's United Rugby Championship clash against Munster on New Year's Day.

"Mack Hansen (shoulder) will undergo surgery today for a dislocated shoulder and is expected to be unavailable for three to four months," Connacht said in a statement.

Hansen has developed into a key player for Ireland head coach Andy Farrell, scoring nine tries in 21 Tests since making his debut in February 2022.

He will also miss Connacht's final two European Champions Cup pool games against Lyon and Bristol.

Meanwhile O'Brien, who came off the bench to replace Hansen when the Australian-born winger limped out of the World Cup quarter-final defeat by New Zealand, will be unavailable for a number of months with a neck injury, his side Leinster said. 

Although the 27-year-old O'Brien has only won eight Irish caps to date, his versatility across the back three has made him an important part of Farrell's plans

Leinster added that lock James Ryan was withdrawn before last week's URC defeat by Ulster as a precaution and will be further assessed this week before a final decision is made on his availability for European Cup action this weekend.

Six Nations champions Ireland kick off their campaign against France in Marseille on February 2.