Here's the deal: After you get that IFF thing and it's installed and something fun happens on board the Normandy, you'll be told that the Omega 4 relay is ready when you are.
Save before, so you can always go back to that save and do more loyalty missions and get more XP and whatever (or if you screw up en route to the "No One Left Behind" achievement).
This achievement pops during the cut sequence BEFORE you actually do anything, and if you can get Paramour, it'll pop right before this one.