Let me tell you a story. Long ago, in Central Europe, there were two language groups which resembled each other. One was the ancestor of Latin, the other the ancestor of Irish. These language groups had many similar words, like the words for land or sea. They also had a similar word for value or wealth. In Irish, thousands of years later, this was to become luach. In Latin, the word became lucrum, and this later developed into the French lucre, which by the time of Chaucer had been borrowed into the English and was used to mean ‘money’. It was often used with words like foul or filthy to show that wealth was corrupting.
This is how English got the word lucre, as in ‘filthy lucre’. There is no doubt or room for argument about this. The word lucre came from French, which developed out of Latin. The word is a cognate (a cousin, if you like) of the Irish luach. But it isn’t a borrowing from Irish. So why is it in this book? How is it relevant to Cassidy’s theory of Irish influence on English?
Your guess is as good as mine. You just have to bear in mind that Cassidy was completely nuts and that hardly any of the claims made in this book make any sense at all.