To be short about it, Ciaran and his younger brother were born to an Irish woman named Saoirse (said 'seer-sha'), and a Pakistani man they've never met. Odd a combination, yes, but welcome to Rukongai. Lol So father fucked off somewhere and mother eventually died; technically they may not entirely be orphans, but Ciaran just ran on the assumption they are. (Side note, if anyone wants to play dad, go for it. That'd be a trip.)
Brother was a bit too young to manage much of anything, so Ciaran kind of started filling the role of dad, mom, and older brother, all at once. Things were rough, but they had each other, and that was the important part. Most likely, brother followed Ciaran to the Seireitei when he left Rukongai, so he's most likely going to be a shinigami.
Now, there's not much set in stone here. The only thing is, younger brother probably has the same Ciaran-like inability to lie and also probably has his hot temper, too. Beyond that, s'all up to you. We can banter and brainstorm, or just post together and see what happens. Ciaran is, and has been, dating Kurogane Iname, one of the big-name noble guys in the Seireitei, for quite some time now, they ended up attending the Academy at the same time and got to know each other that way, so that's a thing to know.
I got nothing here, except Ciaran hit kind of a financial low trying to help a friend out of trouble, and ended up doing escort services for a while. Shit sometimes leads to other shit, and so one day, some vaguely familiar woman shows up on his doorstep, hands him a baby, says, "It's yours," and walks away.
(Who knows, maybe it was a drunken escapade. Certainly, Ciaran really doesn't remember, though this incident did kinda nudge Iname into moving in with him because clearly he wasn't doing so hot on his own.)
So now he's got a baby! Well, the good news is, raising younger brother did make it so he kinda knows how to deal with a baby, at least. Things were probably kind of difficult, for a while, financially, maybe they never really had the prettiest furniture or the nicest curtains or the cleanest carpet, but Ciaran did his best. Eventually, Iname got really bull-headed stubborn (and you kinda gotta be with Ciaran) and insisted on helping out financially, and then later moved in. So then kid had two dads. Iname and Ciaran very obviously care a lot about each other, and while they may occasionally have disagreements or one or the other have a bad day, they're very supportive of each other. Likewise, they're probably very supportive of kiddo.
And you know, Ciaran's pretty much of the belief that a kid's gotta do kid shit, fuck up, and learn the hard way sometimes, so he's the sort of parent that will warn you not to do a thing, explain why, but whether you do it or not's up to you; and if you get yourself in Some Shit, you know Ciaran would rather know so he can fix it instead of hiding it, so he's always made effort to be sure kiddo knows they can talk to him, and has tried maintaining very open communication with them.
Kiddo can be any gender, Ciaran'll love'em anyway. They're probably about 200 something by now. Ciaran's real passion is actually cooking, so being fed real, home-cooked food, is probably very standard with kiddo, and they may be oddly picky about what they eat, now. xD