It's all down to spelling really- a common greeting in Norhern England is 'How's the crack?'
its origin is not clear, but the 'craic' version is invented Gaelicism as you say- no harm in it of course, I think Sean O Riada (John Reid) used the 'craic' version?

It's akin to the way many Irish people do the same with Irish names. There are people of Irish ethnicity called Martin, brought up in the UK who develop a Gaelic persona & become Mairtin- no harm in it, again!