Just returned from Icmeler, flew both ways with Monarch, You can only checkin online 28 days before your flight, so I did mine for the outbound 4 weeks before and for the return 4 weeks before, however I did have initially issues with the return, although I tried 28 days prior I did it in the morning and it wouldn't let me, I tried in the afternoon and it worked, this was due to the fact that my return flight was in the afternoon so the checkin is not strictly 28 days before but 671hrs 59 mins before the scheduled time of your flight.
At
Dalaman there is a online bag drop desk as well but all the other desks were also manned and there was very little queuing at any desk, this was 2.5 hours before my flight.
Dave