To get finicky about the two statements -
'irregular settlements' has the connotation that something is irregular with the settlements themselves, and would totally neglect the irregular pace of construction.
'unplanned settlements' would be those settlements that had no prior planning and appeared spontaneously.
I think the best adjective for what you are trying to say would be 'sporadic settlements' - that word specifically attacks the issue that these settlements are popping up in an unplanned fashion.
Hope it helps! :)