Names are always the toughest for me to come up with when preparing to work on a novel.
The name Lauren Halloran is good but when I say it over and over, it seems it should be O'Halloran but Lauren O'Halloran doesn't sound right either to me. Probably the "in" sound at the end of both names.
As for a middle name with those two names I would say Lauren Marie Halloran because that breaks up the "in" sound.
If you want to keep the Halloran name then what about changing the first name to something like Lacey or Amanda.
Amanda Leigh Halloran
Lauren Marie Halloran
I think with a last name like Halloran, I think you really want to stay away from middle names like Lynn, Ann, or any name like that.
Just my suggestions lol. The final call is up to you. Maybe someone else will offer better suggestions.