Ireland win would be enough to go straight to World Cup as Iceland lose to Portugal
Portugal's 4-1 win over Iceland after extra-time in Pacos de Ferreira means that a victory for Ireland in 120 minutes against Scotland will see Vera Pauw's side qualify directly for the 2023 World Cup next summer.
In the other play-off, Switzerland beat Wales 2-1 after extra-time.
A Carole Costa penalty in the 55th minute put Portugal ahead, following a foul which saw Aslaug Munda Gunnlaugsdottir see red.
However the 10-woman Iceland side equalised through defender Glodis Viggosdottir just four minutes later.
With no more goals in normal time, the match went to extra-time, with Portugal scoring three through Diana Silva, Tatiana Pinto and Francisca Nazareth.
In Zurich, Rhiannon Roberts put the lower-ranked Wales ahead against Switzerland, but Ramona Bachmann equalised just before half-time.
A scoreless second half saw the game go to extra-time, before Fabienne Humm scored a winner in the 121st minute, to put the Swiss through to the finals in New Zealand and Australia next summer.
Portugal must await the result in Hampden to see if they qualify directly, with a Scotland win by less than three goals more than likely required for them to go straight to the finals.
A win on penalties would not be enough for either Ireland or Scotland to leapfrog the Portuguese, which would mean a place in the inter-confederation play-off early next year.