It's probably teak, as that seems to be the most common wood used in marine applications. You can check with a couple of your neighbors to be sure. I would fill the holes with dowel and yes, I'd make it the same wood, as different woods have different expansion properties. It may not make a difference if you are going to varnish it, as it will not be exposed to water. On that note, I would varnish the rails. That's a personal preference and some might prefer oil; very different look. More maintenance involved with oil. Unless you're covering your rails after boat use, you should use a varnish that has high UV protection. 1015 Captain's and 2015 Flagship by Z-spar both have excellent UV protection. They are both costly, so if you're not leaving the rails exposed to the sun all the time, then you can go with Man 'o war varnish. If the stuff you already have is open and old, consider getting new stuff, as I've read elsewhere that there is a definite shelf life to varnish.?.
Reading about good techniques for applying varnish can be found elsewhere and should most definitely be consulted before you get started.
Good luck with your work.
Sun and Moon