[name_m]Jonah[/name_m] [name_u]Dorian[/name_u] by far. [name_m]Quinlan[/name_m] D’alu does nothing for me, and I think [name_m]Jonah[/name_m] and [name_u]Brennan[/name_u] make for a good sibset.
[name_m]Quinlan[/name_m]; [name_m]Jonah[/name_m] is overused at the moment in my opinion. [name_m]Quinlan[/name_m] sounds so much fresher and definitely matches the modern but not “trendy” sound of [name_u]Brennan[/name_u] better.
If you plan on having more children and don’t want to box yourself into the “only Irish surname” corner, I would encourage you to choose [name_m]Jonah[/name_m]. I’m not a fan of names ending in the same sound (in your case, [name_u]Brennan[/name_u] and [name_m]Quinlan[/name_m]). [name_u]Brennan[/name_u] and [name_m]Jonah[/name_m] are different styles and may be unexpected but they’re not so different that that they feel completely mismatched for siblings.