Ironman and phone

I haven't raced an IM in 5 years, but at that time, phones were "officially" banned.  That didn't stop a lot of athletes from carrying and using them.  

By now, you should have a pretty good estimate of how much time each leg will take (as well as your own goal times).  Your girlfriend should know these times and find places to watch you around these times.  If you expect a 1:15 swim, she might crowd around the swim exit starting at 1:10 and watch for you until she sees you or 1:25 at which time she might assume she missed seeing you.  Same on the bike.  Take a page from the race photographers, and have her set up to see you on an uphill area (where you'll be riding 18-23 km/h) rather than a downhill (where you might be riding 30+ km/h).  

Races often have multiple timing mats and will update those results in near real time.  (E.g. if the bike route is 2 loops, they may have a timing mat at mile 20 and another near transition.  You'd cross these mats at mile 20, 56, 76, and 112 and your online results get updated within a minute or two of crossing the mat.)  Same with the run.  

You might also look at the last part of the run route.  Sometimes, they put a little ending loop in the last 2 miles so a spectator can see you with 2 miles to go and walk to the finish line as you're "running" there.  

Several years ago, there were services that offered "athlete trackers" using GPS.  You could rent them for the race ($25?) and your spectators could subscribe to the service to track you.  

YES - the improved Ironman Tracker app (free) is eerily accurate, according to my family who was on-site at the last full IM I did. It estimates and predicts your location based on elapsed time between the timing mats you cross. The only issue is that it's not guaranteed to work, so let your family know that it's not "live GPS" but simply an estimation.