Britian financially costs them about seven billion pounds a year. They see it as a financial burden on them and also having an unstable region on their doorstep. Of course they want to get rid of it, Strategically they can cut their expenditure and re-organise their military to more needed roles in afghanistan.
If the troubles were to kick of again uner a united ireland. Irish troops have been involved in peace keeping operations around the world(kosovo, Chad and lebanon) and are more than capable of stability.