They're fictional names and I rather doubt they were intended as connected to any Hebrew meanings but their form does point in that direction. Lirael could be taken from liron 'my song/joy' and the ending -el (Elohim 'God') to mean 'God is my joy'. Sabriel could relate to the Sabra plant, a prickly pear, native Israeli's are often named Sabra, supposed to represent a prickly exterior but warm heart. Again, adding the - el meaning would give you something like 'cactus of God'.
The books are great creations and I feel sure Garth Nix intended them to echo of existing names but not to be etymologically connected or tracable - as is sensible with fantasy fiction.
You find them mentioned on angel sites, Sabriel in particular. I remain unconvinced. Four angels are mentioned by name in the Bible and I'm sticking with them ;o)
There's a close modern Hebrew name - Sariel. It's male, from the words sar 'prince/minister' + el and means 'prince of God' or 'minister of God' (in the sense of a cabinet minister, not one who ministers to God). Sabi can also mean 'stop, rest' and there's Sadir 'order' which could be contracted into Sadriel 'order of God'. Hope that helps some!