Back to the question: ce/ad (that / is what I'm using to represent the si/ne fada, a longer version of the acute accent) goes over the e in ce/ad, meaning "100".

Cead, with no accent, means permission.

Just a small clarification - the fada (si/ne fada means literally "long stretch", and the phrase is usually shortened to "fada") doesn't *quite* lengthen the vowel sound, but changes it slightly. For example cead is pronounced cyadd, but ce/ad is pronounced cayd.