The Leicester City defender scored a rare goal but lamented costly mistakes at the back after Croatia netted three goals in the Group C encounter
The 27-year-old, who has amassed 29 caps for his country, became Ireland's first goalscorer of Euro 2012 after nodding home an Aiden McGeady cross on 19 minutes - but the Leicester City man admitted he was disappointed with how his team performed in the defensive areas of the pitch.
"We conceded sloppy goals," he told ITV.
"It's unlike us, because usually we're pretty tight."
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Goals from Mario Mandzukic and Nikica Jelavic saw Ireland trail the Croats by two goals to one at the break, but St Ledger believes conceding the third goal so soon after half-time was the killer blow.
"Obviously we knew in the second half that we had to come out and that the next goal was important." he added.
"Obviously, they got it and when you're 3-1 down against a side like Croatia it's always going to be tough."