I like Nora, which I've known on some Irish immigrants, more than Neely, but it ends in -a?
I think Maeve could work well depending on how strong your M-aversion is? Maeve is relatively simple to pronounce, too, and flatters Brenna without being "more of the same" iykwim. I also like Moira which feels very Irish to me, but it's also very vintage and not everyone's taste.
Deirdre is one I love but my Irish-Aussie husband finds it too old.
A couple pages from a UK site that might be helpful, or not:
I did see some goodies but I maybe have a higher tolerance for weird due to being used to it - my husband is carrying on about how he wants Niamh and Siobhan for girls and Eoin for a boy.
What about Keely? Keely Jane would be adorable with brenna claire
I know a Riley and Brennan. G/B sibset.
Logan and Delaney also fit your rules.
I thought Bellamy was french.
Anyways, I like Neely a lot. Kelly is nice too although it's traditionally masculine in Ireland.