I dont think there are any Mexican or Tex-Mexican restaurants in Seoul. But then again my last time being in Seoul was back in when Rockets beat the crap out of Magic. Maybe there are some kind of fusion type of food close to Tex-Mexican food there. Do Outback Steakhouse sells fajita?
Of course. I don't know the restaurant that serve only fajita. but i heard "Bennigance" ' fajita is very delicious. "bennigance" is located in mok-dong. i'm not sure you already know or not, you can find easily where "bennigance" is if you go to its website. and you can also have fajita at T.G.I. FRIDAY too.
I was there last month and I saw TGIFriday's all over the damned place. I'm sure you can get them there , but "good" is probably a stretch.