can I just access a standard int
routine like in dos
If you mean something like DOS' int 21h, then it's int 80h in Linux.
If you mean direct IRQ handling etc. from user program, then no, you can't. DOS is an unprotected OS where you do what you want with the hardware and memory. Linux, as any modern OS, has several levels of security, one of which is restricting user mode programs from *direct* hardware access. You can though go into kernel mode by writing a driver and do what you need including servicing IRQs, accessing I/O ports etc.
does,say, ubuntu linux have
mouse reading functions 'built in' that can be accessed from the plain old bash console
Most devices in Linux are represented by their corresponding device files. These are usually located in /dev directory.
The files which correspond to mice and keyboards usually are in /dev/input directory.