The fullback has signed a new contract to keep him at the club through to 2018.
Cardiff boss Malky Mackay said: "Declan is another one of our youngsters coming through and he has had a little taste of first-team action and a little taste of international action which is great for the football club and for him.
"He should be proud of the progress he has made and there is plenty still to come. He is a young talent we want to make sure we have got here long term because we have a lot of belief in him."
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