Borrowing from one language to another is an interesting phenomenon and something which linguists have studied in depth. I am not one of the linguists who have studied it closely, so I can only offer a rough guide to what words and phrases are borrowed and how and why they are borrowed.
Firstly, I should point out that where two words are similar in meaning and appearance in two languages, this does not necessarily mean that one is a borrowing from the other. This is a basic mistake which Cassidy makes because of his lack of general education and common sense. In some cases, there are words which are completely unconnected which happen to look and sound the same. We have already mentioned the case of daor in Irish and dear in English, both of which mean expensive but have completely different histories. Linguists have lots of examples of these, like the Aboriginal language Mbabaram where the word for dog is dog, or the fact that bad is the Persian for bad but again, these words are unrelated. These are known as ‘false cognates’.
Many other similarities are explained by the words being cognates. A cognate is a word which is found in a similar form in different languages because the languages both derive from the same root, not because one language borrowed the word from the other. For example, almost all the languages of Europe and Asia are related and belong to the same big language family, called Indo-European by linguists. The word tír (land) in Irish is related to the word tierra in Spanish and terre in French because all of these words go back to the same Indo-European root, not because one branch has borrowed from another.
Cognates form patterns because the same sound changes have happened to sounds in the same circumstances in all the words of the language. Thus a trainee historical linguist might notice that German Wasser is equivalent to water in English, that essen in German is eat in English, and come up with a theory that where there is a ss between two vowels in German, this will be a t sound in English. Then the linguist might try finding other examples to confirm or refute the hypothesis, such as besser and better. Thus cognates are often easily recognisable and demonstrable because they fit these patterns of sound change.
And then again, we need to look at the historical circumstances to see which way the borrowing is likely to go. If a word is found in Latin and in Irish, it is much more likely that it is a borrowing from Latin into Irish rather than the other way round, because Latin was the language of religion and learning in Ireland and throughout Europe for nearly a thousand years, and Irish has many words of Latin origin, both directly from Latin and through French. Thus words like marc are borrowings into Irish, not borrowings from Irish into other languages as Cassidy suggests.
There are also constraints on what is borrowed. Far and away the majority of borrowings are single words rather than phrases and they are predominantly nouns. They are sometimes changed slightly to make them accord with the sound system of the target language, but mostly they are borrowed and used as they are in the source language. Thus galore sounds like go leor, shebeen sounds like síbín and both these expressions are used in the same way in Irish and in English.
Where phrases are borrowed, they tend to be phrases that are used and found together frequently in the source language. Thus we hear things like je ne sais quoi in English, or bete noir or Sturm und Drang. Cassidy’s practice of finding an English word which (in his opinion) has no known origin and then inventing an improbable origin for it by stitching together words that have no place being together in Irish and are completely unattested is not good practice. For example, the word helter-skelter, which Cassidy claims comes from áilteoir scaoilte or ‘a loose-limbed trickster.’ I have no idea what the supposed connection is between loose-limbed tricksters and any of the usual meanings of helter-skelter in English. I would have thought that Californians would have had more than enough of crazy people attributing bizarre meanings to that word, but apparently there are still plenty of mugs out there who are prepared to believe anything.