There are two different IR towers. RIS 2.0 comes with a USB tower, while the older sets (RIS 1.0 and RIS 1.5) included a serial tower.
If you have a USB equipped Macintosh, then the USB tower can be plugged directly into your computer. However, there is still the issue of getting appropriate driver software for the tower.
Lego has developed a driver for the tower that runs under Mac OS 9. The driver is bundled with Robolab. Lego has also made the driver available for download as part of the Macintosh SDK which can be found at http://mindstorms.lego.com/mac/default.asp. There is currently no support from Lego for Mac OS X.
The Mac OS X version of NQC does not depend on the Lego driver, and supports the USB IR tower directly.
If your Macintosh is old enough to have a serial port, then you'll just need an appropriate cable to connect the tower to your computer. LEGO includes a cable in the RIS 1.0/1.5 set, but it is for the PC. Most Macintoshes use a Mini-8 DIN connector as opposed to the DB-9 serial connector found on most PC's. You'll need to supply an appropriate adapter - something that has a Mac style connector on one end, and a male 9 pin RS-232 connector on the other. Another option is to use a Mac to RS-232 cable (sometimes called a modem cable) along with a DB-25 to DB-9 adapter. (One of these adapters came in my Mindstorms set even though the box indicated otherwise.) More detailed cable information can be found here.
Most Macs, however, have have USB rather than the traditional RS-422 Macintosh serial ports. Because of this, there isn't a simple cable to connect the IR tower to an iMac - you will need to purchase a USB-serial adapter. Personally, I use the Keyspan Twin adapter (USA-28X). I have also heard positive reports on using the Keyspan PDA adapter (USA-19) and Xircom PortGear USB adapter. There have been negative reports on using Palm USB adapter. If the adapter has a DB-9 style connector, then you can just use the standard Lego cable between the USB adapter and the IR tower. However, if the adapter has a Macintosh style serial connector then you will need an adapter cable as described above.
At first glance, one might assume that the built in IR port of a Mac could talk with the RCX, but because LEGO uses a custom IR modulation protocol, this is not as easy as it sounds. In theory it is still possible to get a Mac's IR port to talk with the RCX, but I suspect there's a rather large gap between "theory" and "practice". Suffice it to say I have not heard of anyone doing this successfully.