Thank you JohnDoe. I've taken the LivingDNA test in the end as thought it may help untangle the Scottish/Irish side, in addition to double-checking my results. This was also after realising I was getting confused between mtDNA and the x chromosome.
The first match that came up on AncestryDNA was Parent/Child - shared DNA 3,487 cM, across 22 segments. My Dad has been tested as well, he didn't come up but I don't know if his results are shared or not. I ignored the match for the first week or so, then went back started researching into how accurate the findings are, and at that level it's apparently fairly conclusive.
I've been estranged from my mother for over 25 years. I know NPE can occur for different reasons , but I do think if she known she would have told me - she has shared other things that have happened. I don't think it was necessarily infidelity, it sounds like a bit of an overlap at the start of her relationship with my Dad.
The second match that came up was his nephew - shared DNA 953 cM across 39 segments. I'd accidentally left myself visible after my most recent late night search (am not now) and he contacted me yesterday to ask how we are connected. It took me by surprise so I wasn't prepared, and was a bit vague as I couldn't work out if he had looked at shared matches or not.
I've joined an NPE group whilst I try and process this, and I know from reading the posts of others that I'm 'lucky' to find this match and that he is still living. It just strikes me as odd that the two closest matches that came up are people I've not got any connection with.
Sorry for yet another long post.