I am seeing a weird difference between my grammar book (Irish Grammar book from Nollaig Mac Congáil). In this book - lenition of nouns chapter - he says that ALL nouns (sg., pl., masc. fem.) undergo lenition in few cases including mo, do, a (3rd sg. masc). then I got to "my department" = mo roinne and I realised that according to that it should be mo rhoinne. I do not see any exception he lists to apply.
I assume this is not regional thing?