I can see where it would appear ironic. On the surface, it is kind of like getting together and discussing the non-existence of faeries.
On a different level, there is no irony. Religious people have a certain viewpoint based on a number of assumptions. Sometimes, owing to the certainty with which they hold these convictions, they attempt to push their personal beliefs onto others and into the political arena.
Atheists are very concerned about the social and political implications of religion. Some feel that measures must be taken to keep any given
religious morality out of the courts and legislature. To this extent, these discussions can be quite relevant; they might help raise people's awareness and help to change laws.
Atheists are very concerned, for example, when pseudo-religious ideas are presented as fact, and as such should be given equal time to science, or in more extreme cases, should replace science completely.
So, it is only on the surface that such an event seems ironic