MacBain's etymological dictionary of Irish and Scots Gaelic gives 'gile' as another word for 'darling', and gilleen would be a diminutive of that.

Cúisle (with a fada on the 'u' to indicate an oo sound rhyming with 'scoot' or 'hoot') is the Irish for a pulse, a heartbeat, an artery, a vein - it suggests that your love of the person is so deep that they're your heartbeat and your pulse. A chúisle (acooshla) means O heart/pulse/etc. Mo chúisle (muhkooshla) means my pulse/heart/etc. Both can be used to address the person you're talking to.