Hi Michele,
Firstly, apologies for not replying much earlier - I had some computer problems and it's only more recently that I've had a chance to look at my previous e-mails. The Ann Dunn who married 'my' John Dunn in 1826 in St Pancras, London, would not have been the same person as 'your' Ann - as you say that her daughter Bridget was married in 1818 - in Kildare.
However, 'my' John Dunn was a widower when he married in 1826 and I've been unable to trace his first wife. If he was born in 1777, as my research indicates, it would have been possible for him to have married in Ireland and had a daughter who would have been old enough to marry in 1818. Presumably, there's no reason why she couldn't have had the same first name as his second wife - but unfortunately, I don't see any way of my finding out definitely, the details of John's first wife. Of course, 'your' Ann would have had to have died before 1826 for it to have even been a possibility that she was John's first wife.
That's my reasoning anyway, but if there's anything else you can think of, please let me know - as I will, you, if I get any more info - but I'm rather doubtful that I will.
Best wishes,
Peter