This is one of the more obviously crazy claims made in Daniel Cassidy’s atrocious book, How The Irish Invented Slang. Cassidy claims that the verb to scrounge is a back-formation from the word scrounger, which he claims derives from a supposed Irish phrase scrabhadh an tír, meaning ‘scrape the country’. Of course, there is no evidence for the existence of this phrase. It is obviously a Cassidy invention, as it doesn’t make sense in terms of the grammar of the language. It would have to be scrabhadh na tíre (pronounced scrowoo na cheera). Furthermore, scrabhadh is not the most common expression for scraping or scratching. Both scríobadh and scrábadh are far more common and neither of these would sound anything like scrounge, as they have a b in the middle.
Scrounge is thought to be a variant of scrunge, a known English dialect term for stealing. This is a lot more convincing than a made-up phrase meaning ‘scrape the country’.