As someone who lived in Belfast and is married to a Filipina the practicalities of travelling south with someone on a UK visa only is quite simple as it is just a main road and no official border checks anymore for years , but I do have to warn you that it is not gardai who patrol the border roads looking for illegals but more often than not it is the immigration staff themselves supported by guards.
In Dec 2006 I was on a bus going from Belfast to Dublin and it got stopped on the main N1 going into Dundalk and they checked all passports so it can happen trust me.
I would check first before making the journey just to be on the safe side.
Best of luck!
