The Irish phrase mar dhea /mɑr’jæ/, /mɑrə’jæ/ “mor ya” is characteristic of Irish English speech. It’s a sceptical interjection used to cast doubt, dissent or derision (or all three) on whatever phrase or clause precedes it. Mar dhea literally means as were it, i.e., as if it were so.*
Sometimes mar dhea is translated as forsooth, but I’m not sure this is helpful. Better to consider it an ironic insertion similar to As if!, Yeah right!, or a sarcastic indeed or supposedly. Bernard Share, in Slanguage, describes it as an “expression of sardonic disbelief or dissent”, while P. W. Joyce says it’s:
a derisive expression of dissent to drive home the untruthfulness of some assertion or supposition or pretence, something like the English ‘forsooth’, but infinitely stronger [English As We Speak It In Ireland]
Mar dhea has been anglicised in many ways, for example moryah, mor-yah, maryah, mara-ya, maryeah, mauryah, maureeyah, muryaa, moy-ah, and moya. It’s a testament to its popularity in Hiberno-English, the diversity of Irish pronunciation, and the difficulty of finding precise orthographic correspondence between the two tongues.