The irrigation did help, no doubt. I don't want to take all the credit away from my fine generalship, however

I would say that each game is different but if Blue Monkey is having the same experience, it can't just be me
Part of my success was that I expanded like crazy early on, and when I first encountered the Celtiberians they weren't much of a factor. The Byzantines were. However, my victory over the Byzantines created a vacuum which the Celtiberians immediately filled. I was lucky because I'd been forced to engage in a military buildup in order to fight the Byzantines so I wasn't completely helpless when the Celtiberians demanded tribute. I had continued my buildup meantime. In a sense, I was forced to become a military power, but again, the irrigation helped because some of my cities would not have become so large without it - not all had ready access to rivers.
As for war weariness, that did force me out of my first war with the Celtiberians, that and my inability to defeat their invisible units. I combated it with city improvements and by using Tribal Council and Monarchy with their low war weariness and avoiding Mahajanapada with its high. I had already caved in to tribute demands several times (from several people) before I said "no" so I knew war was coming. It was build or die.
Another solution to Harappan expansionism,
if this is a problem, might be to change tribal council so that war weariness is more of a factor, or to prohibit the Harappans from adopting Monarchy as a government type.
On the other hand, it was the aggression of others that forced me into warlike mode. I expanded each time I won a war, gaining a city or two here and a city or two there. My only big, Alexander-like conquests came because I had signed a mutual protection pact with the Gauls and they were helping me against Carthage and I saw that if I dropped out of the war and accepted Carthage's offer of peace that they would destroy the Gauls like they'd destroyed the Nubians and that I would be next, with borders too long and irregular to defend. In essence then, I was forced to go into Alexander mode out of self preservation.
It's funny how computer players can behave, regardless of their characteristics/advantages. One of the races in Embryodead's MEM, I believe it is Novgorod, is expansionist, but in a dozen games I have yet to see them expand beyond one or two cities. They've yet to be anything but a victim. The Germans in that game, on the other hand, have been a factor in each game I've played save one. Obviously, terrain, neighbors, etc, all will have a roll in determining how a civ behaves, but with random maps (as in Anno Domini) that can't be controlled. What I wonder then is if every one is having the same experience with the Harappans...?