Well, in Gaelic, one of the definite articles is the a’ which is used in front of feminine words beginning with the consonants b, c, g, m, and p. It is also used as part of the verbal noun- as in a’ coiseachd, a’ bruidhinn or in this case, a’ dol. This makes it the equivalent of English 'walking', 'speaking', 'going'.
Now, the hyphen has to do with emphasis/stress. Usually in Gaelic, stress falls on the first syllable of a word. The hyphen in the "a-" means the stress is on the second syllable. I think these might help.