...according to confidential files...
http://www.irishtimes.com/news/politics/ban-on-irish-speaking-in-maze-prison-sparked-political-row-1.1639948
"The use of the Irish language in the Maze prison caused a major headache for the Northern Ireland Office in the early 1980s, according to previously confidential files released in Belfast."
Should we be surprised that prisoners were not allowed converse in irish?
For all that was wrong with the regime in those days, personally it's not surprising at all.