(gosh, I thought I'd be the one making a huge paragraph against Marth
)
The majority of my experiences are from the three above posters, so I'll be reiterating what they said.
Marth has two things against Ike: He's quick and all of his attacks are sweetspotted at the tip of his blade. In which case, Ike has two problems: Spacing is a horrible idea for Marth as sweetspotted attacks gives huge damage and higher knockback, which means Ike will have to go up close and personal, which will put him in danger of all of Marth's quick attacks. However, this does not give Marth an advantage. In fact, it gives Ike the advantage.
Rest assured that Marth will use dancing blade if Ike is too close. Marth's goal is to have you at the proper range to sweetspot. Majority of the time, they will end with the Side-B (Regular version) of Dancing Blade since it knocks you back. In which case, shield the attacks then spotdodge the final hit, which gives you the time to jab or grab him. I suggest jabbing as the intention here is to stay up close, but you can also grab and do a d-throw or up-throw to put him in the air and do a U-tilt or U-air(watch out for counter if you do this too much). Feel free to jab-cancel into more jabbing here as the majority of Marths will just take it unless there are more platforms.
If Marth ends dancing blade with Down-B (Triple stab version), then spot-dodging will more than likely not work since the attack is prolonged. You can do two things from here, you can shield the triple stab then go for a grab, or you can roll behind Marth during his stabs and go for jabs. You may even get in an F-tilt if you're quick enough.
Eruption is an excellent edgeguard for Marth. There is only one place Marth can go when recovering: Up. Remember Eruption has a huge hitbox, so it's entirely possible to hit Marth with Eruption when he's going for the ledge, you just have to time when he's going to use Dolphin Slash. Other edgeguarding techniques is F-air (not recommended unless Marth is cornered, if he can't return without Dolphin Slash then chances are he'll air dodge instead), D-air, Aether (only do this is Marth is high above, if you attempt this while Marth is low chances are he'll Up-B to the ledge which'll result in -1 Stock for you), and B-air that could beat Marth's Up-B if the player is not on his toes.
Now lets switch the roles. As said before, Marth will use dancing blade if Ike is too close. If it works, then Ike will need to get back up close before Marth goes on a sweetspot frenzy, but don't do it predictably, that's rather worthless. Whenever this happens, you'll want Marth to approach. Remember that Marth's range isn't too good, but it's good enough that it can whack you around. More than likely, Marth will shorthop. If he does, be prepared for either a F-air (more than likely) or a N-air (not so likely). N-air takes a bit longer to end, but it's at an incredibly fast speed, so Spotdodging can work. If F-air, chances are he's trying to go for a sweetspot knockback. It's predictable as hell, so I actually had a good amount of counters against this, but don't do this too much as a Marth can do fake-outs. It's safer to either shield or spotdodge this since Marth will land in front of you due to his momentum. At that point, you're back to where you started.
A good Marth however will mix up his game. Watch out for his grabs. They have low knockback (except for U-throw), but it's just enough to get you back into sweetspot range, plus unlike Dancing blade, it puts you slightly in the air due to knockback. This can allow Marth to do tilts and smashes, so be weary of those.
If you're in high %, watch out for the Up-B of Dancing Blade (I believe it's called the Prestige), It'll put you really high in the air and can potientially put you in the KO range, but the true purpose of it is to give Marth enough time to setup for more attacks to KO.
In which case, if Ike is off the edge, Marth will give you hell. If you're low, he will counter Aether. If you're above, he will F-air or still counter you. If you're just at the right spot, he will D-air spike you. As said before, use reverse Aether to screw up Marth's counter, but personally, I always just aimed for the ledge instead of trying to get back on the stage. Yes, it's still in danger of Marth's counter, but Marth has to get there first. If he does, it only opens up the use of mindgames. Chances are he'll prepare himself to do it again. Use that opportunity to aim for the stage instead of the ledge.
Just like with every matchup, use Smash attacks sparingly. Tilts are more than enough to send Marth flying. If you see an opening (which will more than likely only be from Dolphin Slash), feel free to punish with an F-Smash or U-Smash and get your free kill. Marth will more than likely use his smash attacks sparingly too to keep them fresh, so always be prepared to spotdodge his smashes.
God... this took like 45 mins to write up.