http://countmeout.ie/

If you don’t leave then the church will continue to count you in their official membership counts, and this helps them to gain addition funding, and maintain power within the local community.

There is a similar website for Brits that want to renounce their baptism, but you still need to jump through a few hoops to get the church to recognise it and remove you from the counts.

http://www.secularism.org.uk/debaptism.html