what u need to do after the flashing lights in the IRS is to set the co-ordinates of your position. to do this pull up the FMC then click on POS for position after deleting the message 'set irs position' or something like that look at the co-ordinate already on the fmc, compare it to the one displayed by FS with 'shift-z' and if they match just copy and paste in the boxes (you may need to do this twice before it sets).