The irony of Dave Singleman's name lies in the way that he was a "single man" or a man like no other. This is ironic because he functions as Willy's inspiration and guide in his career as a salesman. Note the way in which he is described by Willy when he is speaking to Howard:
And old Dave, he'd go up to his room y'understand, put on his green velvet slippers--I'll never forget--and pick up his phone and call the buyers, and without ever leaving his room, at the age of eighty-four, he made his living.
Now, even if we factor in a certain amount of exaggeration into the claims of Willy about Dave Singleman, it is certain that he was an amazing man. Yet, what is so ironic about him is that his name implies that this success was not something that could be emulated by others. What Willy fails to understand is that even though Dave Singleman might have achieved such a great success as a salesman, that success was singular to him and that he was a "single man" in achieving this success. The name merely points towards the way in which Willy is wasting his life trying to achieve success that is unacheivable.
The name Singleman also suggests that the octogenarian is literally single. He is a confirmed bachelor. He has been living in hotel rooms all his life and likes his rootless, homeless lifestyle. He doesn't have Willy's responsibilities or money problems. If Singleman travels on an expense account, he doesn't even have to worry about his own food and shelter, whereas Willy has to support a wife and two children and has been making mortgage payments on a house for much of his working life.
Dave Singleman is also a single, unique example just because he has lived to be eighty-four years old. According to the actuarial tables, his life expectancy when he was born would have been around 53 years. Willy probably met him some thirty years earlier and Singleman was 84 then! Singleman is a poor model for Willy to follow. The main thing that impressed Willy was that a man could still be traveling and selling into his mid-eighties. That has made Willy believe that he doesn't have to worry about the future because he can go on being a traveling salesman long past the usual retirement age. But Willy has different genes. He is already used up at age 63 and ready for the "ash can."
Willy probably never wanted to retire. A lot of men are like that--especially businessmen. All they know is business. They don't have hobbies. They don't read much except for newspapers. They can't imagine just sitting around the house with nothing to do. Many men create a problem for their wives when they retire, either because they are forced to retire or because they expect retirement to be enjoyable. Their wives are used to having the home to themselves during the weekdays. Their husbands are just underfoot and interfere with their wives' customary routines. Instead of dreaming of retirement, Willy dreams of living like Dave Singleman, having built up such a large circle of business acquaintances that he can just sit in a hotel room with his slippers on and take orders over the phone.