Nations League champions Portugal have won 18 of their last 21 World Cup qualifying games and are unbeaten in their last six home games against the Republic of Ireland.
They have a talented squad, led by legendary striker and captain Cristiano Ronaldo, who broke Irish hearts four years ago with two memorable headers in World Cup qualifying.
Hallgrimsson admitted that trying to contain Portugal's multiple attacking threats would be “the most challenging thing”, but also called on his side to show a clinical edge to capitalize on any opportunity provided to them.
“We know from their past games that they will have 60-70 per cent possession tomorrow against almost all opponents,” Hallgrimsson added.
“We're going to be defensive a lot of the time. The goal threat from their players is not one player, it's a lot of players. So we can't focus on one player and then only another player will shine. That's going to be the toughest one.
“Then for our forwards to take advantage of the opportunities we get, there may not be a lot of scoring opportunities but we have to maximize our chances.”