This section is a repeat of what is in the the trip report of our mountain biking in Ireland in 2018. I want to put it here as a good source of information for anyone planning to visit Ireland from a country that requires an Irish visa.
We thought that Ireland would be using UK visa for entry into Ireland for Thai passport holders. About a month before the trip we found out from the airlines that Ireland uses its own Irish visa and to get it would take about 8 weeks since the passport would have to be mailed to Dublin !!!
Fortunately in 2016 Ireland announced a special Short stay visa waiver programme ( http://www.inis.gov.ie/en/INIS/Pages/Irish%20Short%20Stay%20Visa%20Waiver%20Programme ) that we can use UK Visa to enter Ireland but would have to first go through UK Immigration. So our Thai members were able to get to Ireland via either London or Manchester to connect to Dublin.