Keller had 202 "overall appearances" for Millwall, with 176 of them coming in league competition. Add that to the 390 appearances Transfermarkt gives for his other European clubs and it's 592 European club appearances, which places him second only to Friedel.
Interesting difference between Friedel and Keller> The both went to Europe to pursue their footballing dreams, only Friedel gained an accent while Keller lost . . .
15 seasons and 415 senior appearances, all of them for one club. That - the one club part - has to be an undisputed all-time record for a Yank Abroad
Are you including league appearances only? Joe Enochs made 359 league appearances for Osnabruck and while he was with St. Pauli early in his career, he never played for their first team. But if you add cup appearances for Enochs, it probably gets a little bit closer (although he won't entirely close the gap). This is a huge gigantic stretch, but if you add the games that Enochs was the head coach of the first team for Osnabruck - he would overtake Dolo. Enochs had a huge footprint on that team. Scott Pittman is around 300 for Livingston, which is still outstanding. He is actually the club's all-time appearance leader. I believe he is the only American to be an appearance leader for a Euro club.
All competitive 1st team games, which includes BL1, BL2, Pokal and UEFA competitions (including 6 EL qualifying games) but does not include RL or Oberliga games
Enochs made 388 first-team appearances in all competitions (BL2, Regional Liga Nord and the Pokal) for Osnabruck, so still a fair few less appearances than Dolo. If you add his 92 games as head coach of Osnabruck, it comes to 480 games which gives Enochs the advantage. It's worth noting that the 3 Liga wasn't set up until the season after Enochs retired, so the RL's were the German 3rd tier while he was playing.
BTW, for those who don't follow YA coaches, Enochs led Jahn Regensburg into BL2 last season and via the really difficult playoff route with the 16th placed BL2 side.
Given your affinity for Ireland, Mr. @The Irish Rover - You'll be surprised that the American club appearance record is in Ireland. This would be Bronx-born John Caufield who played 455 league-play games for Cork City in Ireland. I don't know how many cup or European games he played, but it's a lot as Cork won the cup at least once when he was there. He then was the head coach of Cork City for 7 seasons. I can't see any other American playing so much for one club.