Hmm, níl mé cinnte. Ní léir sin, dar liom.
You can use má, too, with "ba mhaith"
e.g.: Ach má ba mhaith leat an sméar mhullaigh agat chuaigh tú go Meiriceá . (Ó Grianna)

Usually you use dá and conditional only in irreal conditions.
But it is not irreal here, at all. An imperative follows (rather impossiple in irreal conditions)
Conditional mood in ba mhaith is due to the modal meaning (= want) not because of irreality.

I'd translate dá mba mhaith as irreal "If you would like/want (but actually you don't like/want)" which is not meant here.

In case of iarraidh it is:
Má tá tú ag iarraidh tae, tar isteach (not: Dá mbeadh tú ag iarraidh ...)

But on the other side: I don't know.

Took me a bit to figure out what you meant hear by "irreal".
Let me see if I get it.
In order to use dá, the sentence needs to be of the form "If A would happen, then B would happen", for example, "If I had tea, I would give you some."
But in this case, the sentence is of the form "If A is true, then do B", so dá shouldn't be used. Am I on the right track with that?

But in any case, the correct way of saying my original sentence would be
Má ba mhaith leat tae, tar isteach le do thoil

or

Má tá tú ag iarraidh tae, tar isteach le do thoil

Are both of the above acceptable, and which would be most likely used?