Jany, a guess might be that Paisley was hard of hearing... But who knows. He did also call her MacDonald in the initial entry in 1833 where she is a godmother.
The parish records for Westmeath are now online at NLI. Unfortunately, many parished only have records from the 1820 or 1830s on... I am going through them now...
On the plus side, McDunn – if indeed they exist as such, are very rare, just like the first name Frances. So it should be easier to confirm their ID if they show up there.
I've heard rumours that Thomas and Frances also came over, but kept going past Quebec. Why? To where? I wish I knew. Why would Rose stay put along the way? I find no trace in the 1850/1851 censuses.