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 “bhíÂ 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 nóÂ dhó = one or two times, focal nóÂ dhó = a word or two). Honestly, I would not stress too much about all the different situations and just try to learn from example.