The ATR72 is not certified for LCY, so Aer Arann is not a real option.
If 70 seats is really the threshold it would need to be a Q400 or BAe 146/Avro RJ operator - or an existing operator willing to use slots it already holds for an operation with a smaller aircraft (e.g. VLM or Cityjet by Scot when AF/KL cut back on the AMS route once the VG/WX merger is approved)