Glim is an obsolete slang term for a light or a lamp. According to Daniel Cassidy, in his ridiculous book How The Irish Invented Slang, it derives from the Irish gealaim, meaning I lighten, I shine. This is a typical Cassidy claim. When people borrow between languages, they tend to borrow individual words. Occasionally they borrow well-known and often-used phrases. But they don’t borrow random bits of sentences and they borrow the most basic form of the word. In other words, the idea that someone would borrow ‘I light’ rather than just ‘light’ is absurd and the only reason for claiming this is because it wouldn’t sound like glim at all without the I (-aim).
In reality, of course, this is probably just an abbreviated form of the English glimmer.