English. (These notes also apply to some extent to Marian.)
Marion was a medieval diminutive of the French Marie and appears to have been introduced into Britain in Norman times. It is therefore cognate with Marianne. In Gaelic areas it is the Anglicized form of and interchangeable with Muireall and hence a variant of Muriel/Meriel.
Regarding other Gaelic variations, an Australian correspondent [I] writes:
"Merion is more common (than Marion) on Skye and they are all called Sarah (or Sara) when they emigrate or are referred to on their death certificate or on the marriage certificate of a child."
Another Australian correspondent [SM] has studied the names in Kilmuir parish on Skye and found that Marion was one of the most popular in birth records from 1855 to 1900 but far less so since then.
Another correspondent [WBB] has found a mother and daughter, both named Maria, being recorded as Marion in the 1861 Census. These are probably not widely-used variants but should be noted.
A further correspondent [TG] has a reference to Marion being synonymous with, or Anglicized as, Sarah in the parish of Kilmorack in Scotland. It would appear that this is because the Gaelic Mór can be Anglicized as either Marion or Sarah. We have consequently classified Marion and Sarah as synonyms since the equivalence appears to be widespread, in Scotland at least.
A correspondent [ID] has an ancestor in Scourie, Scotland whose baptismal forename was Meran. However she was recorded on census returns as Marion or Marrion while, over the same period, signing herself "Merran ". Also another correspondent [MQ] reports that, in her family and others in Sutherland, Scotland, the spelling variation Merran was common and Mirren could be found further south.
GJ reports that, in the late 16th century, the short form or pet name Maly was used for Marioun (a spelling variant of Marion) in the Kirk Session records of St Andrews, Scotland [transcribed by the Scottish History Society].
TL3 has noted that in the parish registers of Kirkliston, West Lothian, Scotland Marion is consistently spelt as "Merion".