If I recall correctly, they released a single called " Get on your boots" not too long ago. How did that work out in terms of keeping them relevant ? 
Very true 
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But seriously, if they want to stay "relevant", or to become "relevant" again, they need to start appealling to a wider audience, which means writing and releasing strong singles that chart well.
The sad thing is that U2 have been losing their relevancy for the past decade; that is, longer than a lot of people on this forum have been fans. Also, this isn't the first time that the relevancy issue has been mentioned by the band. I remember one of the members saying in an interview during the HTDAAB campaign (can't remember who, or where it was published/aired) that the band were finding it increasingly difficult to stay relevant in the current climate. It was also mentioned after "Rattle & Hum" and again shortly after NLOTH was released. (But not in that order!! )
"ATYCLB" had one ripper of a single (in BD), and three perfectly adequate follow-ups which all charted well. "Bomb" had two strong singles in COBL and SYCMIOYO, and the record buying public (in this neck of the woods, at least) agreed. NLOTH had, in my opinion, none. And the record buying public also agreed.
How many strong singles did JT and AB have? The singles off these albums are instantly recognizable to those who consider themselves fans of music. But the singles from the last three albums (with the exception of Beautiful Day), are not as iconic. There's a pattern emerging here, and it's becoming more apparent with each album that is released.
I think they are (or were) more of a singles band than they would care to admit.
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My 2 cents. 
I'd agree, "relevance", in terms of popular media and airplay, means hit singles. But what makes U2 great, in my opinion, isn't the fact that they have Pride and With Or Without You and One in their catalog. It's the fact that those albums also have A Sort of Homecoming, One Tree Hill, and Until the End of the World on them. Great songs that AREN'T singles, and didn't define the band to a wider audience. Stuff that's not on the greatest hits packages. I say they screw chasing hit singles and "relevance", and pursue what's made them great to begin with. Solid albums, actually ABOUT something, that make you sing, and dance, and THINK. HTDAAB is probably the closest they came to having hit singles with little else to back them up. (NLOTH failed in both regards.) Create great albums, don't worry about chasing the young, hit single market, and they'll have me back.
