Not sure who the specific ad was for - but Ulster Bank and First Active are offering a Tracker rate of ECB plus 0.75 (4.75) on LTV under 80% on investment property.
Also BOSI are offering the same rate if you are borrowing more then 400k.
ICS and BOI also offer this rate on loans over 500k.
Could be a nominal rate where the APR is higher? Could be a time limited discounted tracker rate (a bit of a contradiction but they do exist) after which the margin increases? Could be a misprint? Could be a scam? A mobile number for such an offer wouldn't inspire my confidence.