You need to have strong confidence in your own product to carry advertisements for the competition in key parts of your own website, but this is exactly what Ryanair is doing! After all it makes sense, if you have the cheapest air fares in the market to let the other airlines display your prices next to yours, this is what we can see in the example below, plus they can make some money when someone clicks on one of the adsense ads.
Spot the ads for competing airlines and travel sites on the right
It would be interesting to know what percentage of Ryanair's website visitors end up going to the competition (something that the Flightblogging blog already noted some time ago), but if the ads care still there, and given Ryanair's legendary ability to extract to the latest penny of value from its operations we can be certain that the overall effect has a positive effect on the Ryanair's bottom-line!
Adsense ads are actually placed automatically based on a number of criteria, but you can choose where you place them on your site, and Ryanair has gone us far a placing them in the centre of their main site. That at the time when I checked it while I was writing this post, it was carrying ads for: Eurotunnel!
Spot the Eurotunnel commercial in the central part of the site