Regarding irun, one thing which is useful is "irun -helpall | grep <word of interest". So irun-helpall | grep seed gives:
-seed <seed> Set the seed value for SystemVerilog and Specman
-snseed <seed> Pass seed value to Specman
-svseed <seed> Set SystemVerilog default RNG seed
Numbers are seeds, "random" will be randomly generated seeds.