Chances are, if you've read any of my nodes or been to anywhere north of Birmingham or Leicester (aside from North Wales), but south of Durham, you'll have heard someone use this expression. It is, arguably, the expression that defines "northern" in the popular mind.
Alternatively spelt "eyup," "ayup," or "eyoop," it has many uses, from a greeting - "Eh up lad, how do," - to an expression of curiosity, like in Blackadder - "Eh up, who's this big girl's blouse?" - to something meaning roughly, watch out - "Eh up!!" If you've seen Four Lions, Chris Morris' black comedy about homegrown terrorists, you'll know that Waj, one of the Four, starts his martyrdom video with, "Eh up y'unbelievin' kuffar basterds!" which gets all three of these uses into one go. And, of course, The Macc Lads use it on many an occasion, what with their inimitable Ramones cover "Eh Up, Let's Sup" and even used to have a big banner slung behind them with "EH UP!" on it during live shows.
Also, a former premier of Bosnia and Herzegovina is named Ejup Ganic, and his first name is pronouned in the same way, roughly. But that has nothing to do with anything here.
There seem to be several different derivations of this expression or family thereof. One of them peculiar to Nottingham and environs - "Ayup mi duck!" - is alleged by a BBC correspondent to be a contraction of what the knocker upper in industrial times would say giving alarm calls to his workforce combined with having to avoid a chamber pot. "Are you up, my man?" combined with "DUCK!" as he sees something unmentionable about to land on a passer by from an upstairs window. But I'm pretty sure this is all lies. Indeed, the use of it as a contracted knocker upper's alarm call is slightly undermined by the fact that it's used in rural areas as well. The most convincing one that I've seen is that it's a corruption of the Old Norse (and current Swedish) "se upp" meaning "Watch out!" Given that in the Dark Ages, Yorkshire, Lancashire, Cheshire, the East Midlands, and Humberside (all of which are in, for better or worse, the "Eh up" belt) were ruled by the Vikings, and still have lots of Viking derived place names, indicating heavy Viking settlement, it was probably a Norse expression that the local Engliscmen found themselves learning because it doesn't help to shout "Watch out!" at someone who's about to fall off his horse or whose attention you're trying to garner for whatever reason and them to think, "what is this foreign johnny drivelling about?" Given that the Vikings were in charge of Northern England for a good century or even more, it had plenty of time to become embedded. Then, as the Vikings left, over time it became corrupted into just "Eh up!" It probably took just as long because throughout the Middle Ages and up until affordable mass travel, people generally didn't move about much, hence different dialects becoming embedded.
Nowadays, with easier travel, television, and the internets (especially the latter) taking hold, you're more likely now to meet a real Northerner saying "Eh up!" a bit more rarely. In TV-land it's basically shorthand for "this person is from the North." Also, mass immigration. Just as if you go to London you're more likely to encounter the locals talking in a hideous dialect known as "Jafakean," which is basically one half Cockney and one half Jamaican creole, and always sounds permanently disaffected and a bit, well, poseurish considering that the people who use it, well, the closest they've been to Kingston is the one in Surrey, but Jamaica, being land of reggae, Bob Marley, weed, jerk chicken, and gay-bashing as the national sport, all of which are considered Cool and In right now (Odin help me as to why), finds its mannerisms aped by inner city yoofs and, more likely, middle-class nathans trying to pander to said yoofs. Innit blad. But anyhow. Just as because of this you won't find a real Londoner under 50 who still says "Cor blime, luvva duck, me ole choina!" and the Jafakean accent is taking over (due to the Afro-Carribean immigrant population), in the North, you'll probably find local youths peppering their discourse with stuff lifted from Indian and Pakistani colloquialisms (due to the fact that a lot of South Asian immigrants came over and settled in various mill towns in the 1970s) you probably will find not all that many Northerners under 50 who still say "Eh up!" But they still exist, that's for sure.