Customs at CDG

You will go through immigration in Paris as you would if coming directly from the USA as Ireland is not part of the Schengen Zone. Count on 1 to 3 hours depending on how many flights arrive at the same time and how many agents are working at passport control and then baggage claim if you are checking baggage. You might wait to buy the train ticket until you get to the train station instead of in advance; it happens that the flight will be delayed or canceled. Not unless you get a really cheap train ticket then it is worth the risk

Jazz has good advice -- and 2 hours is not close to enough time. The rule of thumb is 4 hours between an international landing at CDG and a train from Paris. You might be lucky and have an early landing plane, a short immigration line and get to Montparnasse in 2.5 hours or so, but if you buy tickets ahead you need to plan for more typical experiences which will generally take at least 3 hours to get to town.

As noted, Ireland is not part of Schengen; there is also immigration when coming from the UK e.g. London. so you have international immigration on arrival.

Isn't the 4-hour advice mainly for people flying in from North America? With a time cushion for flight delays as well as immigration? This OP's flight is from Dublin so unlikely to be significantly late.

Also, there's at least one TGV direct from CDG to Rennes, bypassing Paris. Look for it here. Might be the easiest way to go if the timing works with your flight.

No less than 4 hours.

Should I plan for more than 2 hours to go through customs, get the bus and arrive at the train station?

Short answer, 'yes'

Long answer, absolutely, positively, without a doubt-'yes'

4 hours minimum...at least 2 hours to get out of the airport (immigration, luggage), an hour at least on the bus depending on traffic, and enough time to get your tickets at Montparnasse. If you have to ride the inter-terminal train at CDG watch out for pickpockets. I had my first experience thwarting a pickpocket there this summer.

Always assume that where there are crowds and all public transport that there are pickpockets and the only defense is to be pickpocket proof i.e. no easily reached valuables. I use a money belt under my clothes in transit with all my stuff and never set the computer bag down at my feet or on an empty seat or whatever. Pickpockets go where the pickings are good and the inter terminal conveyance at CDG is a great place for that as are museums, the metro and anywhere else where there are lines and crowds (line to Notre Dame for examples)

Using Le Bus Direct (or its previous incarnation) I've made it to Montparnasse from CDG on 2 occasions from 2 to 2.5 hours, but I wouldn't count on that. The fact that you're flying from Dublin lessens the chance of the flight being late, but immigration time is a big factor, as is getting baggage if you check any. 4 hours is more realistic, so if you're planning to buy an advance purchase discount train ticket that is nonchangeable/refundable, leave plenty of time.

My experience coming into CDG is arriving from Iceland (also Schengen so passport already stamped) with only a small carry on (no wait for luggage). I've done it several times and noted I'm always right at one hour before I'm on an RER rolling out of CDG. The trip to Montparnasse on RER is about one more hour. I prefer RER because I don't have to worry about road traffic in Paris.

Short story, two hours isn't quite enough time even without immigration or luggage waits. Depending on when you arrive, immigration may be quick or may take up to two hours.

If immigration is a long wait, you may be pushing it in four hours. If you experience a long wait at CDG, I'd skip le Direct Bus and take a cab. It might save you the valuable minutes you will need (but also subject to road traffic).

Should I plan for more than 2 hours to go through customs

Not at all. Customs is a walk-through unless you purchased items to declare.

It took me almost 2 hours this morning from deplaning until the 4 bus to Montparnasse left CDG. Furthermore, I went through the short EU passport line. The non-EU was five times the size.
Revised: light traffic so the bus got to Gare de Lyon in about 40 minutes once it started rolling. Montparnasse is not more than 20 minutes furthermore than the Gare de Lyon, so count 4 hours to catch a train.
However, it is easy to catch that train from CDG directly to Rennes, but when I took it it was not a TGV and took 3 hours.

Should I plan for more than 2 hours to go through customs
Not at all. Customs is a walk-through unless you purchased items to declare.

Oh, you know what OP meant. Kind of nasty to leave this kind of misleading info

Really, you never know. I have both planned not enough time and way too much time. Last time I arrived in Paris at 6AM and was in Montparnasse by 7:30. I would say it is better to wait than to miss your train.

Save tons of time by only having carryon luggage. Can’t stress that enough. You can thank me later. From the time we landed at cdg we were at the rental car counter in on hour this June.