None of that affects that the MttH for the settlement event is 50 years. I get what you mean about the actual average time being likely to be lower when both events are possible, since there are two seperate checks, but that doesn't affect the OP's example at all as it was in Ireland. There are no neighbouring provinces of english culture, so the cultural assimilation event wasn't a factor there at all.
Actually, I was just checking the event again to make sure of this and I noticed cultural assimilation also requires the revolt risk to be under 1%. Since the settlement policy modifier also adds 8% rr then it's unlikely both events will be possible in the same province even if you do have a neighbouring sam culture province, unless you have enough rr reduction to negate this.
Edit: this was in reply to Junuxx, took too long!