In answer to your question, I don't know, but at a guess, pretty easy.

As an example, maybe 5 years ago, I visited Ireland with a Chinese friend. She had not got ILR and at that time I was naive with regards visa regulations. I simply booked my Ferry drove on drove off with my friend, there and back no questions asked.

I have been to Ireland maybe 6/7 times and never been stopped at customs. The other option is to drive up to Stranraer take the Ferry into Northern Ireland and just drive down to the South.

I am not suggesting that you do this, but I'm simply telling you my own experiences.