The OpenSprinkler firmware currently does not use I2C for OSPi at all. There is no hardware conflict — the I2C pins are free to use by any I2C device. You may want to write a small and separate program just to test the I2C functionality, such as using wiringPi to detect MCP23017 to make sure the wiringPi I2C functions are working properly. I have no experience with using wiringPi for I2C so I don’t have any advice. Also, make sure I2C is enabled in your RPi configuration.