By coincidence I've just started reading a book called At Swim Two Boys by Jamie O Neill in which the term Segotia (spelled like that) is used. The character speculates as to where the name comes from, and wonders is there a flower of the same name - from what I can make out so far the character is actually not too bright and has already misused a number of big words, so I'd take this definition with a grain of salt. If there's any further enlightenment further on in the book (I'm only on Chapter 1) I'll let you know.

As for Dubs being slang for toilets, I hadn't heard that one before, but we Dublin people often refer to ourselves as Dubs but also as The Jacks - which is definitely a slang word for toilets, so there seems to be a theme running here.

Instinct tells me that Seo dhuit é is a red herring as a source for the word. The phrase litterally means "Here it is for you", which wouldn't make sense, and the pronunciation of the phrase as Segotia is a bit far fetched as well.