as quick start you can connect ISP1581 to GPIO pins and emulated a the data control address bus. you just have to write some c routines that access the isp1581 registers, by toggling gpio pins) thats easisest for start and quick test.
for real application it would be preferred to write a xilin PLB IP core that talks to isp1581, but software wise it want change anything. You may leave it for later as it usually isnt without problems :( custome PLB peripherals often dont work at the beginning...
same with shared RAM or whatever you decide to use, two options you create a peripheral core, or you use ISE as toplevel, this is also not without caveats, works but only if you do many many step in row and all precise correct. then you can write portion of the design in verilog/vhdl and use microblaze SOC as schematic component in ISE
antti