"Mar sin leat, a bhalaich."
This is not a comment on "Mar sin left" but rather on "a bhalaich", which, to the English ear seems odd to hear on a regular basis. My father used this all the time in greetings and farewells to his male friends - well into his 80s. I used to think of it as the equivalent of the US use of "buddy".
This phrase originated as a response to beannachd leat / leibh, which literally means "blessing with you".
"Mar sin leat / leibh" would traditionally have only been used as a response to a goodbye and translates as "with you also."
This distinction is no longer strictly observed in modern Gaelic, and "both mar sin leat / leibh" and "beannachd leat / leibh" can be used to say goodbye, regardless of who initiated the goodbye.
(From the tips section. I added American quotation marks to make up for the loss of formatting on mobile.)
It’s not just a spelling change for no reason— the spelling change reflects a change in pronunciation. bh- or mh- at the beginning of a word sounds roughly like /v/ in English.
For adjectives such as bheag, or mhath, this change—which is called
lenition—occurs when they are modifying a feminine noun: luch bheag or madainn mhath.
Nouns such as balach, kinship words (piuthar, brathair, seanair), and personal names such as Mòrag receive lenition (a bhalaich, a Mhòrag) when in direct address with ‘a’.
beatha and other nouns receive lenition when they follow a possessive adjective: dachaidh= ‘homeland’; mo dhachaidh = ‘my homeland’. Likewise, beatha = ‘life’; do bheatha = ‘your life.’