The Irish number system is considerably more complicated than most other languages you will find. As Labhrás says, when counting (i.e. with “a” preceding the number) you say “dó” and before a noun it is “dhá”. However, note that when preceded by “an” it becomes “dá”. So in your example “tá dhá mhír sa chlár” = there are two items in the program, if talking about them in the definite “bhí an dá mhír go hiontach” = the two items were excellent. Additionally, if you are saying something like “one or two of ___ ” it becomes “dhó” (ex. uair nó dhó = one or two times, focal nó dhó = a word or two). Honestly, I would not stress too much about all the different situations and just try to learn from example.