IC vs. Marth is evenish, but it's one of Marth's better top 8 matchups IMO.
The simple things to do are DI up against down throw down air and jump out.
Don't try to wall out with only aerials; good ICs will slip through if you have a definitive rhythm.
Down tilt is fantastic for stopping their approaches. It's really difficult to shield grab with the same tricks you do to Sheik. You can also go into short hop or jabs during the IASA frames to make it harder for them to time forward air approaches against repeated down tilts or forward smash approaches against your aerial keep out.
You can just short hop over ice blocks. (Also jab them)
Time your approaches against Ice Block spam with Popo's blocks since that's the one that he controls.
DON'T DO RANDOM FORWARD SMASHES WHILE THEY'RE ON THE GROUND. You will lose a stock for free if they shield it and it's not a tipper. If you ever feel compelled to Forward smash them while on the ground at least put some thought into it and tipper their shield.
Hit confirming forward air > forward smash is really good, especially if they're by the edge of the stage since Nana will usually get hit and Popo will either get hit or shield and slide to ledge.
You can forward air on top of their shield pretty safely and depending on positioning, you can just keep pushing them out. I wouldn't use down air TOO much on their shield, but neutral air is pretty good for covering everything if you're unsure how they're going to come at you.
You generally want to use forward throw if you grab them. It often pushes Nana/Popo away so you don't get forward/down smashed by the other one. You're also less vulnerable in general.
Take opportunities to safely kill Nana, but you have to recognize that Popo will try to stop you. You generally want to make sure Popo is controlled or play to wall them both out. Nana will slowly try to maneuver around you to make it back to Popo, observe carefully how Nana is influenced by your position and you can just keep swatting her away until she can't jump back anymore.
Don't sleep on SoPo. All it takes is one down smash and you're losing 60% or a stock. Even though IC damage is down, it greatly increases his maneuverability. He's going to be harder to hit, and he's going to look to counter every swing you take. I generally like to be more grab and jab heavy in this phase for that reason.
Be patient edgeguarding ICs. Deny the edge at every opportunity, watch and wait for air dodges, and don't overextend. SoPo has a tough time doing an aerial double jumping and recovering, and has a tough time dealing with up air and up tilt trying to land on the stage from above.
I know top ICs have gotten really good at the matchup and it's only going to get worse because they're hitting harder than ever. Always try to wiggle out of grabs, ESPECIALLY if Nana is out of position to force the down throw before hand offs can happen. If you're playing correctly, it should be difficult to get clean grabs on you. Nana should almost always be out of position or hit if you ever get grabbed.
If you do get grabbed by the edge... I think DIing up against hand offs makes it a little bit harder than DIing away. If they're on though... it's a stock so if you want to do stupid laggy stuff that gets you punished, try to do it in center stage at low percentages.
http://www.youtube.com/watch?v=b2Qy4tOjeTc
This match is from 3 years ago, I was super rusty (I still am relatively), but the general theory around the matchup still applies now. Since Wobbles is my roommate now and he's one of the best players in the world, I think I'm still doing pretty well in the match up today. He's gotten a lot better (obviously) and I'd say that now I probably win 70ish% of our matches depending on the day.
I like to reset my position a lot, especially in this matchup. They obviously don't want you to, especially if they just hit you. So that's where you need to be careful and not get sniped by back airs and down smashes. It will happen; you just gotta shake it off and focus on doing your job. This is one of the only high tier matchups where Marth can really keep someone out effectively. ICs just don't have quite the range or attack speed that other characters have.
FD isn't a bad stage for Marth in this matchup. Yoshi's is double edged but more in your favor. DL is only bad if you screw up your edgeguards and give ICs more chances or you fall behind and have to do long approaches against them. I'd say every neutral is fine against them, so I'd just figure out what stage that IC likes and remove it. FoD is a little too random for me, so I might ban that or DL depending on how I'm feeling.
I'm pretty confident that anyone that applies my strats and has better handles with Marth than me will beat any IC. You don't need to be fancy; you just have to control your character.
React and Deny.™
Last note: This is just my current take on the matchup currently. I used to think the matchup was even more in Marth's favor, almost a hard counter. ICs are hitting much harder than they used to, and they've gotten a lot better at timing approaches and slipping through Marth's wall outs. I'm pretty open to the idea this matchup will become even or even disadvantageous in the near future, but I still think it's Marth favored mainly because I've done really well against every IC I've ever played, and I still do well against Wobbles, who is undoubtedly an overall better player than me. I'm just a specialist.
