To get technical for a minute "A chuisle" is the vocative case and "mo chuisle" is the genative. These have been anglicised as Acushla meaning "Oh pulse/heartbeat" and Macushla meaning "My pulse/heartbeat".

As Malcolm says above these would have been taken as generic Irish terms of endearment and used by many songwriters as such.

There was a well known landmark in Dublin called the Macushla which started out as a ballroom and later became a Bingo Hall. It is now the site of offices of the Irish Gas Board "Bórd Gáis".

Michael Marra has an interesting song set in Dublin called "to please Macushla" which is set in Dublin, and is well worth a listen.

Machree is an anglaicsation of "Mo Chroí" (modern Irish spelling - used to be "mo Cridhe") which means "My Heart" or "of My heart".