A couple of points that are worth addressing, I think.
1. Atheism is only irrational in precisely the same way that a lack of belief in mermaids, unicorns, fairies,hobbits, elves etc. is irrational.Atheism is a rational response to an extraordinary claim - that there is/was an omnipotent, supernatural being that created the universe and all that is in it.
2. Agnosticism seems on the surface to be a more reasonable position to take. - People are drawn to that position by the perception that it is a less dogmatic, more reasonable position to take. At its heart is the precept that humanity cannot either prove or disprove gods existence, and that therefore each position is equally valid.
This is the problem with agnosticism.There are hundreds of variants of godhood, spread across many diferent religions, almost all of which have their own variant of a creation story.In all of these though, it is gods hand that reaches out and affects the universe.So, if events could be identified that have no identifiable natural cause, one could suppose that this was the action of god.
It seems to me that any rational, reasonable person would think that an omniscient,supernatural being that can reach out and manipulate the universe, respond to intercessory prayer, and looks after immortal souls - all these are extraordinary claims. The rational response to an extraordinary claim is to ask for the extraordinary evidence - and this is where religion falls down - because that evidence is nowhere to be seen.
Miracles? Weeping statues,apparitions, stigmata - plenty have been recorded, but the vast majority are poorly documented and the evidence for them is poor.There is, for example, currently a guy in India facing arrest for showing that the "tears" of a weeping statue was actually sewage from a blocked drain - so the catholic church in India press charges of "deliberately hurting religious feelings".
Geocentricity was an important tenet of religion, defined by scripture - Disproven by Copernicus and Galileo, but not officially dropped by the catholic church until mid to late 1800s? Not until the 1990s did the Pope offer a papal pardon to Galileo.
This illustrates an important part of the debate. Science, rationality, reason, changes with evidence - religion attempts to "reinterpret" the evidence to support scripture, and will dogmatically cling to scriptural precepts instead.
Science has been a powerful tool in advancing human development.It has, gradually and remorselessly, refuted all those things claimed by religion to be proof of gods existence, supernatural creation and continual monitoring and oversight.
Whilst we cannot explicitly prove gods existence or non existence at the moment,our knowledge expands with each passing generation - so there is no reason to suppose that at some time in the future we could arrive at a definitive test.What we can show, is that each of those claims held up by the religious as proof of god is gradually eroded.
Given these conditions, it would seem the only rational position to adopt is atheism.Indeed, for theists, faith is a badge of honour - and faith is belief in the absence of evidence.