If you are attending iFM 2018 you should verify whether you need a visa to enter Eire/Ireland, and apply for one if you do.
The page at http://www.inis.gov.ie/en/INIS/Pages/check-irish-visa will guide you through a series of steps to determine whether you need a visa depending on your country of origin. When asked “What do you want to do in Ireland?“, choose “Conference or event” and follow the instructions. At the end of the procedure you should get a visa transaction number, which you should note carefully.
When requesting an invitation letter, you should have already registered for the conference and should include a copy of the visa transaction number. Invitation letter requests should be submitted via this link, clearly indicating all the necessary information.
Note: Ireland is part of the EU, so EU/CH citizens do not need a visa. However, Ireland is not part of the Schengen area, so if you are from outside EU/CH and have a Schengen visa, you may still need to apply for an Irish visa. Please check the website above if you are uncertain.