I thought it was interesting (and not altogether surprising) that Joyce doesn't use the traditional spelling of "meow" (or "miaow") to indicate Leopold Bloom's cat is making a noise. Instead, he uses such spellings as "Mkgnao!" and "Mrkgnao!" to make the noise.
I'd thought that perhaps "meow" hadn't been standardized yet, but in fact the spelling has been around since the early 17th century. So Joyce's use of his alternate spellings is a deliberate choice, not an invention of a sound before convention standardized it.
With that in mind, Joyce's spelling accomplishes three things. First, it gives the cat a little more personality than the average literary feline. Second (hand in hand with the first), it allows some more variation in the noises it makes. When sounded out, the noises actually sound fairly realistic. And third...
...it makes the reader scratch his head and think, "what the hell?"
I can't help but wonder which of these effects was most important to Joyce.