How did Scottish Gaelic develop a future meaning from its inherited present tense?

Scottish Gaelic developed a future meaning from its inherited present tense as a result of its verbal system being remolded on the lines of the British language, which originally lacked a future tense.1

In this process, the inherited present tense form shifted to primarily express future meaning. To compensate for this change and express present time, the language utilizes a construction consisting of the present-tense form of the substantive verb combined with the preposition a(ig) and the verbal noun. For example, while the Old Irish téit in ben ("the woman goes") evolved into the Scottish Gaelic théid a’ bhean ("the woman will go"), the present continuous sense is now conveyed through the construction thà an bhean a’ dol ("the woman goes" or "the woman is going").1