Remember what happened to their Father?Erskine Childers and his son Erskine Robert (the second Protestant President of Ireland)
There was a considerable amount of misogyny back then, believe it of not. When a Protestant of either gender wanted to marry a Catholic their children had to be raised Catholic.No doubt this is a factor. Although why the stat restricts itself to 15% of 'male' Protestants I'm not sure? Surely the same outcome for female Protestants marrying Catholic men?
It was one of the many factors which made this country an alien place for Protestants after Independence.But you are talking about a Catholic and Protestant history. What has that got to do with Irish language?
Irish Nationalism had become much more entwine with a culturally Catholic identity over the preceding decades but yes, when Michael Collins and his party partitioned the Island, after the IRA made it an inevitability, a chasm opened up between Protestant and Catholic Irish people as "Protestant" became a byword for Northern Unionism.The point being, Irish Protestant Nationalism is very much an integral part of Irelands struggle for independence from Westminster rule.
Partition drove a knife through that sentiment.