Cast spell if ready if not cast other spell macro?

[Chaltione]

I suggest something like:

#showtooltip
/castsequence reset=ctrl/alt/shift Ice Barrier, Mana shield, Mana shield

with as many repetitions of mana shield as you wish. This will keep using mana shield after the Ice Barrier, and you can shift/alt/ctrl click the macro to start over at ice barrier (remove any modifier and slashes as applies).