"scissors" is one of these strange English words that are always used in plural (because they consist of a pair of something, like pants, shorts, pliers). In order to unambiguously convey a singular meaning (which they want you to do hear), you have to use "a pair of scissors".
A scissors cuts is always grammatically incorrect in English. If you just use the "scissors" without "a pair of", it is a plural subject, and article and verb need to agree with it: Scissors cut, or: The scissors cut.
Listen to the Munster speaker at teanglann - he uses a broad s (ess sound) rather than a slender s (sh sound) for both of the "s"s in "siosúr".
Normally, an "s" that is next to the slender vowels "e" and "i" will be pronounced with an "sh" sound, and an "s" that is next to broad vowels ("a", "o" and "u") will have an "ess" sound. But there are a small number of exceptions, especially when a word has been borrowed, where speakers who are family with the word "in the wider world" may retain more of the original pronunciation, whereas people who are reading the word rather than hearing, will follow the rules and come up with a different pronunciation. Another well known and similar example is "seisiún" mentioned above