interest only/arrears

I have two questions

I have a mortgage of 300,000, only 25,000 paid off.
If i was to go on interest only for a year, approxamitely how much extra would i be paying back in the long run? (no tracker, fixed rate, AIB)

Also, if we were to go in arrears, how long does it stay on your cresit history?

Thanks in advance,
Rupert