I don't think its perversity. Some people have been given a hard time in school over pronouncing 'th' as 'd' and become so self conscious about it that they over compensate to the etent that they get things wrong at times. Most would be unaware that they are doing it.

It's difficult to generalise about Irish accents as there are strong regional variations. In general in Dublin there is a tendency to drop the final t in a word so that 'that' becomes 'tha' or 'da'. On the other hand you often meet Dublin people who very makedly pronounce the 't' which I think again comes from tryong to sound educated.