Ireland (or Eire) is an independent sovereign state, as opposed to Northern Ireland (also known as Ulster), which is a constituent country of the United Kingdom, along with England, Scotland, and Wales.
Ireland is not a member of the Schengen Agreement, so a Schengen visa or residence permit will not allow you to visit Ireland without a visa. A valid UK visa does not entitle you to travel to Ireland without a visa unless you qualify for the short stay visa waiver program.
This visa is intended for a brief stay in Ireland. The visa is valid for one entry into the country for a period of up to 90 days from the date of issue.
This is the visa intended for a short stay in Ireland. The visa is valid for multiple entries to the state up to a maximum of 5 years from the date of issue.
Unless your arrival flight and onward connecting flight are both in Terminal 2 between 0400 and 1600, you will need to go landside to connect. This necessitates obtaining permission to enter the state (you will need to have the appropriate Irish entry visa if you are visa required for entry to Ireland). You will also need a transit Irish visa if you will be transiting through another airport.
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