I use a pool pump for a similar application to yours. This pump I picked up at the side of the road as someone had removed their pool and was throwing it away. It was fully functional and works great for my purpose, in fact it was a little too strong for my purposes so I put a ball valve on the output so I could throttle it down a bit, works great. Not sure about using it to augment your irrigation system; I can say this though I know someone who built a filtration system for 150 fish tanks and uses a similar pool filter to move the water, something that I imagine to be somewhat more demanding than running 5 or 6 sprinkler heads at what 1.3-2.5 GPM each? Something to consider.

Here is a very simple solution - The rain barrel pump.
only $85
http://www.rainbarrelpump.com

Here is a link that might be useful: Rain Barrel Pump

If you're just looking to periodically move water from one place to another, the above pump works; referred to as a Pony Pump and selling in Canada at Princess Auto for $99.
Even simpler though is a pump that attaches onto any portable drill. The "self-priming drill pump" sells for $10 and I can tell you first hand it works great. Best of all, with a cordless drill, you can use it anywhere, including as a safety firehose running from a nearby pond or stream to a fire out in the bush.

Here is a link that might be useful: self-priming drill pump

Go to harborfreight.com and type in "water pump" in the search box.
Kenny

You could always check out Pondmaster or Danner Mag Drive pumps, for affordable in-line or submersible little monster pumps, which never die.

I have a 500 GPH, which I have used for years in various hydroponic gardening systems, and it could be used for your purpose, but as mentioned...moving water up, about, along the various runs, and having the correct GPH/PSI required to do so, can be a bit of a mathmatical nightmare for DIYers. ;)

I use a submersible RIGID 1/6th HP submersible pump [looks exactly like this http://www.amazon.com/Wayne-RUP160-6-Horsepower-Oilless-Submersible/dp/B0009X8O2E]from Home Depot in my 2 barrel system, just drop it in with a garden hose attached. It sounds impressive 3000GPH, ya whatever, it produces about 20psi so not regular water pressure but it works and takes an hour to drain 110 gallons at 20psi though a sprinkler.
I was using it to power the back and forth fan sprinkler, but the screen or jets would get plugged with small debris, and it just wasn't powerful enough for the tis, tis, tis, pulse sprinkler.

I did however find a solution at a recent home show. http://www.rainforestsprinklers.com
They were selling it to people with poor water pressure in the country, and who use pumps from a well and that sort of thing. It seemed perfect, so I bought the Rainforest 24' Riser Sprinkler for $20 bucks or so. Works really nicely covering a 20ft circle with 20psi. It's impressive with the regular water supply it was probably spinning 3 times faster, with a bit of mist created and a 35ft circle, but at 20psi it was a gentle rainfall and ran for 1 hour on the 2x 55 gallon drums full of water didn't get plugged once, has a fairly large hole for the water, and it was always tiny debris already ground up by the pump that came down the hose and plugged the fan sprinkler anyways, I just don't use one of those rubber o ring mesh screens with the rain barrel anymore. I'm happy with the 24' riser, and might even go with the 36' if you have a big yard, but I think the ground level one would be just too small of a water pattern with such low pressure.

Anyways I was so impressed to finally be able to water my lawn without stopping to clean out the sprinkler.... I wrote the sprinkler company this was their reply 'Thanks for letting me know how well our sprinklers worked with your system and the rain barrels, we have been experimenting ourselves with them and managed to get a 15 ft diameter by just running connected to a rain barrel that was approx 12 ft higher than the sprinkler with no pump. We get many emails from people in places like Texas that couldn't run a sprinkler at all on their pressure and ours worked great for them and saved them hours of hand watering.'

I would add to really shop around if your looking for a pump, most places want to sell you a pond pump that has no pressure, or tiny hoses, or a HUGE expensive jet pump that can produce 80psi and clean out your wallet, it's hard to find something in the middle cheap that takes garden hose. If I did it again maybe I would buy a bit higher HP pump, or a sump pump with a float if I could find one cheap. I think the higher HP would drain the barrels faster, but also give more PSI to run a regular sprinkler.

How did you set up your barrels to symphony out both with your pump?