This is currently avaible for Chinese and Indian citizens and will be available of Thai passport holders at some point during 2015.

Governments agree British-Irish joint visa scheme

BBC News, 6 October 2014 Last updated at 15:33

A scheme allowing visitors to the UK and the Republic of Ireland to use the same short-stay visa has been ratified by the British and Irish governments.

http://www.bbc.com/news/uk-northern-ireland-29509552

UK visit visa holders from certain countries, including Thailand, have for some time been able to enter the Republic without an Irish visa; provided they did so via the UK.

Good to see that the British government are at last reciprocating.

What would be even better, though is if both governments agreed to let holders of other categories of visa, e.g. settlement, do the same.

Although, of course, family members of EEA nationals already have that right under the EEA regulations.