Shill is apparently a fairground term for someone who masquerades as a member of the audience to encourage people to buy things. From some posts I have seen on the internet, I strongly suspect that Daniel Cassidy knew all too well what the word shill means. According to Cassidy in his crazy book, How The Irish Invented Slang, the word shill derives from the Irish síol, meaning a seed. On the face of it, this is reasonable enough, though a serious etymological researcher would be worried by the difference in the vowel – why wouldn’t the word have been taken into English as sheel? As I’ve said before, most borrowed words are a lot closer to the source word than Cassidy suggests, because at some stage in the past a person or a group of people who spoke Irish threw the word or phrase into a sentence in the English language and it was heard and picked up by people who didn’t know Irish. If there are difficult sounds or combinations of sounds (like dl in Irish dlúth or dlí) then these will be ironed out and changed because the native English speaker won’t be able to handle them. But síol presents no problems for a native English speaker, so why would the vowel be shortened so that it rhymes with hill?
The other problem, which is far more important, is that shill seems to derive from an earlier fairground word shillaber which means the same thing as shill, a plant in an audience. Nobody knows where shillaber comes from and none of the suggestions are particularly strong but it doesn’t sound Irish.
Once again, Cassidy’s claim is demonstrably wrong.