For each of these I recommend the exercise of first trying to spot on your own what is ironic about each of these anti-religious billboards. Then afterwards see if my observations match your own. I am sure you will discover many things that I missed:
1. Keep the Merry! Dump the Myth
Irony: Santa Claus is in part a mythologized version of St. Nicholas, who I understand was generous, but was neither rotund, nor always jolly (he apparently punched the heretic Arius at the Council of Nicea). As for having a sleigh and wearing furry red attire, not so much either. On the other hand, Jesus of Nazareth does appear to be a real historical figure (whatever you think about his identity), and the events depicted above do appear to be real and historical. Thus, what our dear "American Atheists" seem to be saying is in fact the opposite of what they intended. Atheists are always talking about guarding reason against those dreadful and dangerous fairy tales, but here it appears they are the ones upholding the myth, while Christians are defending the reality.
Secondly, a very practical point. If you dispose of the source of your celebration, then there can hardly be any substantial reason to celebrate. It's like saying "Keep the body, but cut out the heart." Now an atheist might say that pagans celebrated the winter solstice long before Christians, and that Christians merely plagiarized it. No one is saying that pagans had no celebrations- the question is, what provides more cause for celebration, the movement (or behavior) of a celestial body, or a raucous and jubilant birthday party that goes on for days and days?
2. There's Probably No God...
Irony: When I first looked at this billboard I thought that perhaps Dunkin Donuts sponsored it due to the colors, but I cannot confirm that. The first irony here is the phrase "God probably doesn't exist." Assuming that this sign comes from an atheist group, I have to marvel at their lack of faith in, well, their lack of faith. If I said to a fellow Christian that God probably exists, he would in all likelihood tell me, and rightly so, that I am a poor Christian, or at best, that I lack faith. Thus, I say the same to this atheist; "You sir, are a poor excuse for an atheist, and if you belong to a club of them, then you should be excommunicated.
The second irony is the implication that someone who believes in God simply goes around burning with anxiety about doing the right (or wrong) thing. This therefore proves that religion is altogether a bad proposition. I might argue, however, that it is better to burn with anxiety about doing the right thing than to feel nothing if you don't. Serial killers and mass murderers are quite content with their handiwork. Sociopaths are unmoved by their trail of misery. Is this the type of "enjoyment of life" that an atheist prefers?
3. This Season, Celebrate Reason!
Irony: The biggest problem for atheists is that they never get around to first principles. Contrary to what they assert, they are the ones who believe in castles in the air, for it is they who would argue that everything exists without any original "ground" of being. So, for instance, in the rhetorical statement above they declare that the story of the Magi is false because it is a myth. What never occurs to them to ask is where these myths come from? And secondly, do these so called myths have any basis in reality? I for one do not have trouble imagining that a set of Magi/astrologers followed the stars in order to find some truth. Is that not what astrologers do? For that matter, is that not what scientists do? The atheist cannot prove that it didn't happen, so he does the only "rational" thing... dismiss it without further investigation. "Why isn't it true? Because it's a dumb fairy tale!" Very academic if you ask me. But the real irony here is the notion that our time would be better spent "celebrating reason" as opposed to Jesus. While we're at it, why not throw a party for oxygen, or what about the vitreous humor. I can already hear the kids brimming with enthusiasm! I am all for celebrating "reason," "oxygen", the "brain", or whatever else causes one to marvel, but what seems to me the very opposite of reason is the notion that we should use reason to prove that there is no reason at all. In other words, if reason is really worth celebrating, then should it not lead us to something that we can truly rejoice over (like hope and/or meaning). Instead we are told that we should exult over the fact that we are little more than an accident, a mutation, a mistake, a tiny speck of nothing floating for a milli-second in a cold and loveless universe, on a planet which is utterly indifferent- if not hostile- to our presence. Yeah, that's worth celebrating!
4. Choice on Earth
Irony: This is not technically an atheist billboard, but I felt that its anti-Christmas theme was somehow apropos of the general idea of this post. Some forms off rhetorical irony are so sinister that you wonder if the person is not simply mocking you with it. In any case, the result of this one is the logical equivalent of comparing a child being born to a child being dismembered. But I suppose in an upside down world the death of a child really is something to chirp about (as you can see above). When it comes to Planned Parenthood, nothing shocks me anymore. Along with their "choice on earth" campaign, they also offer a very special holiday gift certificate- kind of like the ones you get from Barnes and Noble. However, in this case you do not use it to acquire books, but rather, in the spirit of Christmas, you march yourself into one of those magical Planned Parenthood clinics and avail yourself of a previously payed-for abortion. The biggest irony of all is the fact that there would be no "Choice on Earth" campaign were Planned Parenthood to have had their way in the first place, for they would certainly have counseled the Mother of our Lord to do the only right and responsible thing (especially considering Joseph's dubious paternity, as well as their unfortunate financial circumstances), thereby canceling Christmas forever.
5. I Do Not Find in Christianity One Redeeming Feature
6. I Can Be Good Without God
Irony: It has always struck me as more than a little odd when people try to put a happy face on something that would not necessarily inspire true happiness. For instance, it's like having a picture of a smiling woman saying "I had an abortion and I'm quite pleased about it". Something just doesn't connect. In this case, the disconnect comes from the fact that in this billboard we have a smiling man, who is apparently doing so because God isn't there. For this reason, atheism seems to me a little like worshipping a doughnut hole. Woo-hoo, unicorns don't exist! Why am I grinning from ear to ear? Because I just found out that there are no such things as fairy Godmothers or guardian angels! Let's take a picture of me smiling about it. I guess what I'm saying is that it just comes off as a bit disingenuous. As for that accompanying phrase "I can be good without God" it reminds me of that song "Missing You" by John Waite; "I ain't missing you at all..." Are you sure about that, 'cause you sound like you're protesting a little too much....
The second irony has to do with the lack of coherence in the atheistic argument. I would never argue that an atheist can't be kind or friendly. The question is what is the basis of this kindness. If it's a natural inclination to be kind, then I would simply say that they do it, not for goodness sake, but because it makes them feel good. But even if they do do it for the sake of pure altruism, why do they feel compelled, as pure Darwinian primates, to defend it in the first place? This would seem to militate against the view that the only virtue in evolution is survival. The other question is on what basis does one even call something good if one is an atheist? Goodness cannot be empirically verified, therefore it would seem to me that it can only be explained in one of two ways: as a superstition, or a religious sensibility.
7. All Religions are Fairytales
Irony: Yes, I would have to agree... if by fairy tale, you mean that all religions (or at least most) seek to affirm what appears to be the deepest longing of the human heart (i.e. immortal gladness). Interesting rationale: because our hearts long for something, that therefore proves, beyond a shadow of a doubt, that it doesn't exist.
8. Sadistic God; Useless Savior
Irony: 4 quick comments on each of these accusations; 1) Is it "simply reasonable" to begin your criticism of Christianity with an ad hominem attack? "Christianity is sadistic and evil". That's not an argument, that's an accusation. What if I were to say, "All atheists are stupid!" ? Would you regard that as a rhetorically astute statement? Of course not! The truth is the bottom of this billboard should not read "Atheism: Simply Reasonable"; it should read, "Atheism: Simply Emotional"; 2) I am pleased to see that atheists are using a common Catholic argument to point out the problem with denominationalism (i.e. 30,000+ "versions" of the truth). I wholeheartedly agree with this criticism; 3) It is always comforting to know that atheists place such a high value on love, and equally disconcerting to know that Christians, whose God is defined as such, are the true source of all the hatred in the world. That's right, Jesus' command to "love your enemies" is the real reason violence persists in our world today. Maybe we should amend the Scriptures so that instead of it saying that "God is love", it should say; "Atheism is love!" 4) I'm sorry, I have to admit that if a clear image of Jesus appeared in my morning toast, I would have to call it a miracle... or at minimum, remarkable. Maybe that makes me credulous, but I have to go with the numbers here.