Republic of Ireland striker Robbie Keane has admitted that he still does not know why his side underperformed at the European Championships last summer.

Ireland lost all three of their group games against Croatia, Spain and Italy and were far short of their best even accounting for the quality of opposition.

They will have an opportunity to gain a small measure of revenge against Spain when they take on the eventual champions in the early hours of Wednesday morning, and Keane is hoping that his side will provide a much sterner test this time.

"It was difficult for everyone because of the manner in which we were beaten by teams, whereas in the past, that has never been the case," he said.

"The way we set our stall out to play against these teams and how we are always a very, very hard team to beat, for whatever reason at the Euros, it just didn't happen for us.

"In saying that, we were playing against the two finalists, Italy and Spain, at that time, the best teams. It was always going to be difficult for us, but the way we were beaten was disappointing for everybody."

Ireland will face Spain in the international friendly at New York's Yankee Stadium.