"Chan eil fios agam."
Translation:I do not know.
It is historically correct, and was more common, at least until recently. I fear that too many people learning grammar think 'what is the h doing there', and take it out.
The reason for the h is really quite weird. Firstly, eil, which does not look like tha actually comes from an old verb meaning 'observe'. So chan eil fhios agam actually calques as '[one] does not observe knowledge at me'. That makes the fhios the direct object of the verb and that would have been lenited many centuries ago.