First off, let's examine why you get the aliasing violation warnings.
Aliasing rules simply say that you can only access an object through its own type, its signed / unsigned variant type, or through a character type (char, signed char, unsigned char).
C says violating aliasing rules invokes undefined behavior (so don't!).
In this line of your program:
unsigned int received_size = ntohl (*((unsigned int*)dcc->incoming_buf));
although the elements of the incoming_buf array are of type char, you are accessing them as unsigned int. Indeed the result of the dereference operator in the expression *((unsigned int*)dcc->incoming_buf) is of unsigned int type.
This is a violation of the aliasing rules, because you only have the right to access elements of incoming_buf array through (see rules summary above!) char, signed char or unsigned char.
Notice you have exactly the same aliasing issue in your second culprit:
*((unsigned int*)dcc->outgoing_buf) = htonl (dcc->file_confirm_offset);
You access the char elements of outgoing_buf through unsigned int, so it's an aliasing violation.
Proposed solution
To fix your issue, you could try to have the elements of your arrays directly defined in the type you want to access:
unsigned int incoming_buf[LIBIRC_DCC_BUFFER_SIZE / sizeof (unsigned int)];
unsigned int outgoing_buf[LIBIRC_DCC_BUFFER_SIZE / sizeof (unsigned int)];
(By the way the width of unsigned int is implementation defined, so you should consider using uint32_t if your program assumes unsigned int is 32-bit).
This way you could store unsigned int objects in your array without violating the aliasing rules by accessing the element through the type char, like this:
*((char *) outgoing_buf) = expr_of_type_char;
or
char_lvalue = *((char *) incoming_buf);
