Northern Ireland is part of the UK (the United Kingdom of Great Britain and Northern Ireland). It shares the island with the Republic of Ireland (or simply Ireland), but Northern Ireland and Ireland are different countries.
If you plan to fly to Belfast directly or travel through Great Britain (e.g., through London), you may need a UK visa.
If you require a visa to visit the UK, and you plan to travel to Belfast via Dublin or visit the Republic of Ireland from Belfast, then you will likely also need a separate visa for the Republic of Ireland.
Note that while both the UK and Ireland are members of the EU, neither are in the Schengen area. One cannot visit these countries with a Schengen visa alone.
Letters of invitation for XXX ICPIG 2011 may be requested from firstname.lastname@example.org. Letters will only be issued to confirmed invited speakers or participants with an accepted abstract who have paid the registration fee. In the latter case, a letter can only be issued after 23rd May 2011. If requesting a letter, please make it clear whether an electronic or hard copy, or both, are required.