Getting my single entry in Dublin Ireland. Will enter at Suvarnabhumi airport. Will I be asked to show 20000 baht, even though I will be in possession of my single entry visa??

It is highly unlikely.

 

They only tend to ask those making frequent/long visits for tourism.

Have the equivalent in foreign currency. 

As stated funds can be in baht or any currency or travellers cheques. Having holiday in los and intending to use ATM means desire to donate to banking system. 

Side note. I realize op is from Dublin.... I ran into Scottish guy last week in Saigon trying to exchange Scottish pound. No luck.

Bring cash in accepted currency to Thai money exchange. 

Worth bringing in some of your holiday money in cash (guess it'll be euros if you're coming from Ireland), if for no other reason than to avoid the excessive bank charges levied at both ends. Don't change money at the airport when you arrive other than for immediate taxi costs, much better rates downtown. This will give you a benchmark of the rates you'll get locally, much higher than the tourist rates quoted in the UK newspapers.

https://www.superrichthailand.com/#!/en/exchange

32 minutes ago, lamyai3 said:

Don't change money at the airport when you arrive other than for immediate taxi costs, much better rates downtown.

In the basement in the area behind the Airport link are money exchangers like Superrich, 15 satang per USD/EUR worse than the Superrich Headquarter, so unless somebody wants to exchange many thousand USD / EUR it's ok to just exchange there. Of course don't exchange at the Kasikorn / SCB booths, they are a ripoff.

Edited July 26, 20196 yr by jackdd

This is what  I have adopted now. every time I leave Thailand, I keep at least 5K BHT. When I enter, I get 500 USD or Euro (if coming from Europe) from the departure airport ATM and I exchange it once inside Thailand  and put the money in my Bangkok bank. Never been asked for the money in 15 years but always be prepared. 

Edited July 26, 20196 yr by onera1961

11 hours ago, DrJack54 said:

I realize op is from Dublin.... I ran into Scottish guy last week in Saigon trying to exchange Scottish pound. No luck.

Well he will likely be bringing Euros so no problem!????