Re "Nice to have"---acknowledged, but. . . . The junior programmers I know will still be intimidated by that list. And the intermediate/senior programmers will see it and wonder why you're offering so little. So it seems like the ad is conflicted.

If someone really needs those skills to do the job, then perhaps you can transform a negative into a positive: "New hire will receive on-the-job training time and budget in Mongo, Python, and Hadoop." That would attract a lot of people! And probably adding training is cheaper than raising the salary.

Once your ad has a clear message, perhaps you can use it in more targeted venues than SO. Imagine a CS student at Dublin University seeing a flier on their department bulletin board:

    Junior C# Developer Wanted
    Excellent Salary
    Will Train in Python, Mongo, Hadoop

That's something you might even be able to send to a couple professors and ask if they could pass it on to any outstanding seniors.

Even better would be to s/Excellent Salary/€35 - 45k/.