By half time in the friendly at the Met Life Stadium in New Jersey the Republic were trailing 3-0.
The second half started much more brightly for the Irish and just five minutes in, James McClean pulled one back.
The former Derry City winger turned the defender before smashing past the Portugese keeper.
As Ireland changed their formation to try and reduce the deficit Portugal hit them with two further goals.
Shane Duffy who made his senior debut for Ireland in last Friday's draw with Costa Rica came on to replace John Walters just after the hour mark.
As the Irish head home, Portugal are off to to the World Cup were they kick off the competition against Germany on Monday the 16th June.