Hi

[Boreasos]

In my view irrationality is simply the lack of rational thought, where a rational thought is one that is backed up by evidence and/or valid logic. An irrational belief would therefore be one that is believed even when there is no demonstrable evidence to support it, or there is demonstrable evidence that contradicts it.

Just so that I am sure of your position: let's say I hold belief B, and there exists demonstrable evidence and/or a valid logical argument for B, but I am not aware of that evidence or argument. Would B still be "rational"? And is any argument that is not rational automatically "irrational", or is there an intermediate "arational" category (like moral, immoral and amoral)?

B would still be rational(as it is demonstrable) , but you would not be rational in holding a belief in it if you were unaware of any evidence to support it(since from your point of view you are holding a belief despite the lack of any evidence that supports it).
As far as I define it myself, anything that is not rational is irrational, there is no arationality.

Under this definition I therefore classify theism as an irrational belief, since there is no demonstrable evidence to support it.

When you say there is no evidence, do you mean that you, personally, are not aware of any evidence, or that there is actually no evidence?

I cannot of course say for absolute certain that there is no evidence, for I do not hold the entirety of human knowledge. However, to my knowledge there is actually no evidence to support it.