Security issues aside, it is worth bearing in mind Eircom's recent agreement with some representatives of the music industry over access to copyrighted material online. Eircom have agreed to implement a policy of "3 strikes and you are out" if one of their customers is identified as accessing such copyrighted material. The first two incidents result in a warning, the third results in your Eircom broadband service being withdrawn. If someone uses your wireless router to access such material, you will be held responsible as things currently stand.
There remain lots of questions about whether Eircom can really enforce this in law. Some people claim that you could argue that your wireless network was not secured and therefore you are not responsible for the online antics of anyone using your broadband connection, others claim that this is not a defence that holds any water. There is also the issue that many Eircom routers remain vulnerable to the widely publicised hack that allows the wireless encryption key to be derived from the wireless network name (where the wireless router configuration is the one that is was delivered to you with), which some argue effectively removes any responsibility from the home user for their wireless network being "hacked" and used by others. I haven't seen any mention of Eircom applying this new policy as yet but obviously that doesn't mean that they haven't, or that they won't. So, right now, no-one knows whether it is a policy that is truly enforceable or even whether Eircom will actively try to enforce it. But it is one more thing to consider if you opt to leave your wireless network unsecured (i.e. no shared key/password required to use it), or choose to use an extremely low leve of security (WEP).
Incidentally, if security is a true concern for you then WPA2 is a better alternative to WPA, but support for WPA2 isn't yet as widespread as support for WPA so fewer products support it right now. WPA2 provides stronger encrpytion than WPA - specifically, WPA2 supports AES for wireless encryption while WPA supports TKIP. Several months ago a weakness in TKIP was exposed which, although it hasn't been turned into an effective exploit as yet, it certainly paves the way for that to happen at some point (maybe next month, maybe next decade - no-one can predict when). Something that clouds the issue is that WPA actually supports both TKIP and AES, but that is an early implementation of AES, while WPA2 supports both TKIP (for backward-compatibility) and AES also, but generally speaking when people refer to WPA they are talking about TKIP and when referring to WPA2 they are talking about AES. For maximum security of your wireless traffic, opt for WPA2/AES whenever you can.