I think this term has been discussed in Mudcat before. a-cushla from Gaelic "a chuisle", o my love. There are many terms of endearment; this one comes from "cuisle" a pulse, as in a-cushla machree, "a chuisle, mo chroi" - o, pulse of my heart

I don't know of a site that would give you transliterations. For vocabulary for Irish speakers and learners, try www.beo.ie or www.acmhain.ie