Hi,
First post here, so be gentle!
Although I haven't availed of the following service myself it may be of interest to those who want a VIP arrival/departure service:
http://www.dublinairport.com/at-airport/vip/index.html
They do provide a shower room although their facilities/service obviously come at a premium - €200 for a return journey. Apart from the shower facilities, they take your bags, check you in, personal pre-board security check etc.
If it's just a shower that is required, I believe the Clarion are offering overnight rates from €89 - in fact there seems to be a 'rate war' between the airport hotels at the moment due to the new Bewleys and Hilton - so this would be more economical?