In fact there is nothing irrevocable about "triggering Brexit". We shouldn't be fooled by a metaphor like 'triggering". All Article 50 means is giving notice of an intention to leave the EU, with at least a two year period of negotiations, which can only begin at that point. That two years can be extended if that is agreed by both sides.

But there is nothing to stop a government which has given such notice of its intention from anandoning it, and cancelling the whole thing. There is no need to seek permission from the rest of the EU for doing that. It seems pretty evident that one reason for insisting on at least two years delay is to give time forsecond thoughts, or a change of government.