you will need a multi entry visa. I did the same last year. belfast-dublin-india, back same route. if you apply visa via post you will get it in a week.
i had inquired with Irish embassy, quoting from their reply:
Reply from
visamail@justice.ie :
The first step of the visa application process is the completion of an
online application form, which can be accessed via the following link:
https://www.visas.inis.gov.ie. The signed two page summary sheet, along
with all the required supporting documentation must then be submitted to
the Visa Office/Embassy/Honorary Consulate as indicated on the summary
sheet, contacts for which can be found on:
http://www.dfa.ie/home/index.aspx?id=285
Information when making a visa application can be found on the Irish
Naturalisation and Immigration Services website:
http://www.inis.gov.ie/en/INIS/Pages/Ir ... nformation