The Republic will be aware of the outcome of the other group game involving Armenia and Hungary before they kick off against Portugal.
“It’ll have a significance but in terms of how we’re going to approach the game I don’t think it will have too much initially,” explained O’Shea.
“We have to focus on what we have to do against Portugal and to get that result that we need.
“They [the crowd] have a massive effect on how we perform and the energy they can give us, but we have to help them with that, we have to give them something to cheer about. It works both ways. When we’re both pulling the same direction we’re hard to stop.”
O’Shea described Thursday’s encounter with one of Europe’s heavyweights as “a massive game”.
“We have to be tight, we have to be aggressive, clearing crosses, the usual stuff.
“It’s a World Cup qualification game at home against a top seed.
“It’s why we do it. It’s why you’re in football for these scenarios, to be facing a good opposition, but also to know you can be part of a fantastic victory for your country. So that’s the key bit behind it and it’s why we do it, it’s why we love it.”
Jamie McGrath has also been called up by Hallgrimsson for the upcoming matches.
Hibernian midfielder McGrath is a replacement for Bristol City’s Mark Sykes who sustained a gash on his leg during last week’s 1-0 defeat by Blackburn Rovers.
McGrath won his last cap as a substitute in a 2-1 away win against Finland in October 2024.
Updated Republic of Ireland squad
Goalkeepers: Caoimhín Kelleher (Brentford), Gavin Bazunu (Southampton), Mark Travers (Everton)
Defenders: Seamus Coleman (Everton), Jake O’Brien (Everton), Dara O’Shea (Ipswich Town), Nathan Collins (Brentford), Liam Scales (Celtic), John Egan (Hull City), Ryan Manning (Southampton), Jimmy Dunne (Queens Park Rangers), Kevin O’Toole (New York City)
Midfielders: Josh Cullen (Burnley), Jack Taylor (Ipswich Town), Jayson Molumby (West Bromwich Albion), Finn Azaz (Southampton), Conor Coventry (Charlton Athletic), Jamie McGrath (Hibernian), Andrew Moran (Los Angeles FC)
Attackers: Troy Parrott (AZ Alkmaar), Adam Idah (Swansea City), Mikey Johnston (West Bromwich Albion), Chiedozie Ogbene (Sheffield United), Festy Ebosele (Istanbul Basaksehir), Johnny Kenny (Celtic).