out of what you’re considering, I particularly like [name_m]Cillian[/name_m] and [name_m]Ramsay[/name_m] as a sibling to [name_m]Lochlan[/name_m], and between those, I think [name_m]Cillian[/name_m] [name_m]Arthur[/name_m] flows the best.
some suggestions!
Balor
[name_m]Caolán[/name_m] (prn. KEE-lan) - similar sound to [name_m]Keegan[/name_m] without resembling it in spelling
[name_m]Conan[/name_m] - meaning “little wolf,” balances out the soft/sweet with a bit of mischief? 
[name_m]Desmond[/name_m]
[name_m]Donovan[/name_m]
[name_m]Duane[/name_m]
[name_m]Ennis[/name_m]
[name_m]Finnian[/name_m] - possible alternate to [name_m]Finn[/name_m] by itself
[name_m]Flynn[/name_m]
[name_m]Gael[/name_m]
Macdara
[name_m]Malcolm[/name_m]
[name_f]Naoise[/name_f] (prn. NEE-shuh)
[name_m]Nevan[/name_m]
[name_m]Oisín[/name_m]
[name_m]Rory[/name_m]