David
McDowell lives in Portrush. It is safe to say he has a lot of links experience. When a golfer comes home for the summer from a US college he would play a lot of tournament links golf eg McDowell in 2000 played tournaments at Lahinch, 2 at Portrush, Castlerock, Royal Dublin, Carnoustie that I can think of off the top of my head.
So guys like Donald, Casey etc would still have played a lot of links golf while in college in the US. Even Poulter who turned pro at a young age would have played plenty of links golf.
The question you asked was how many golfers on the European Tour are experienced links golfers, all the GB&I golfers are, if playing lots of links golf growing up and 2-4 tournaments a year on tour (the Dutch Open is played from time to time on links) means being an experienced links golfer, which I think it does.
The big GB&I amateur tournaments (The Amateur, St. Andrew's Links, Lytham Trophy, Irish, English, Scottish, Welsh Amateur, Brabazon, etc.) are played on links courses and have a big entry from Europe so the vast majority of top class European amateurs and hence eventual players on the European Tour have significant experience on links courses as well. Yes, plenty of courses on tour have a TPC feel but these guys are the some of the best players in the world and can adapt their games with little or no hassle.