My experience with BoI a few years ago was that they would not give top-up funds except for renovations. There is a request for plans, work progress reports, valuation reports, etc.
They will not care that you are abroad once you keep paying your mortgage. You can even change the correspondence address to your address abroad. However if you ring them up and voluntarily advise them that the house is now let they seek to charge you a BTL rate.
Also bear in mind that you may be generating a CGT liability now that the property is let.