I've read in the Caighdeán Oifigiúl about fractions

An eleventh (1/11) is aondéagú, alright.
They give an alternative they call "an focal cuid leis an orduimhir".
But instread of normal an t-aonú cuid déag (cuid + ordinal number an t-aonú ... déag) they use an t-aondéagú cuid which I think is strange and redundant.

Is this really used?

I don't think so. It looks like one of those inventions made by people thinking in English.
Maybe in some cases it's easier to use than the genuine number, if you do maths in Irish. But apart from school pupils, I don't think many people do maths in Irish...

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