it's not at all uncommon for one AV to find something that another one doesnt. have you contacted trendmicro to ask them to investigate whether its a false positive? don't just assume it is. altho, if it was something within the basic mIRC (as downloaded from triggering it, then seems like everyone with mIRC who uses housecall would get the same results

ParaBrat @#mIRCAide DALnet