If your epubs are not DRM'd, you can just add them to the Books section of your itunes library. iBooks will play them just fine on an iPhone or iPad.
It's also easy to add epub books to the Kobo iOS app for reading on an iPhone or iPad.
1. Plug in your iPhone or iPad, as you'll be syncing it with iTunes.
2. Go to iTunes, and click on your device.
3. Across the top of the screen, choose Apps.
4. If the Kobo app is installed, then look at the bottom on the screen in iTunes. You'll see a section called "File Sharing" and a list of apps that allow you to load files.
5. Click Kobo, and the box on the right will rename to "Kobo Documents".
6. Below the Kobo Documents box is a button called "add", which will allow you to browse your computer and find your ePub files.
Honestly, it sounds more complicated than it is. See my picture below, where I have two books currently loaded onto my iPhone's Kobo app.
Also, if you look at my picture, you'll see I have the Nook and Sony apps ("Reader") -- both of those apps could load ePubs as well. All three apps are very good for reading, highlighting, and notetaking, as is Kindle (but it doesn't display ePub files).
ETA: I just realized I had missed the same explanation above. Oh, well. Maybe the image will be helpful.