Thanks to http://www.mathematik.uni-marburg.de/~schmidtm/brainfuck.php for the C code ofwhich I converted into mirc... painstakingly after realising I did a few things wrong...

It does seem to work, I think its 100% working but I may have implemented somthing incorrectly.

Basic examples work fine so you know...

/bf ++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++.>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.------.--------.>+.>.

Outputs: Hello World!

alias splitchr return $chr(32)
alias maxexecute return 10
alias memorytest {
  var %myCode plornt=Hello my name is plornt;test=2;out>plornt;

}
on *:INPUT:@Brainfuck:{
  if ($lines(datadump.txt)) {
    noop $bf(-continue,$1-,$read(datadump.txt,1),$read(datadump.txt,2),$read(datadump.txt,3),$read(datadump.txt,4))
  }
}
;Very useful alias when things piss you right off.
alias haltbf .remove datadump.txt

alias bf {
  window -e @brainfuck
  if ($1 != -continue) {
    bset -t &bfuck 1 $1-
    bset &mpointer 1 0
    var %spointer 1, %bpointer 1, %kl 0, %len $bvar(&bfuck,0)
  }
  else {
    ;Continue where we left off :P I suppose its easier to set the data first then input the stuff you inputted.
    var %spointer $gettok($3,1,32)
    var %bpointer $gettok($3,2,32)
    bset &bfuck 1 $4
    bset &mpointer 1 $5
    var %input $2 $+ -
    bset -t &mpointer %bpointer $right($left(%input,1),1)
    var %input $right(%input,$calc($len(%input) - 1))
    if ($6) { 
      bset &output 1 $6
    }
    var %len $bvar(&bfuck,0)
  }
  var %time $ctime
  while (%spointer <= %len) {
    if ($calc($ctime - %time) >= $maxexecute) {
      break
      msg $chan BrainF**k code halted: Max execute time of $duration($maxexecute) reached. 
    }
    var %byte = $chr($bvar(&bfuck,%spointer))
    var %mpvar = $bvar(&mpointer,%bpointer)
    if (%byte == +) bset &mpointer %bpointer $calc(%mpvar + 1)
    elseif (%byte == -) bset &mpointer %bpointer $calc(%mpvar - 1) 
    elseif (%byte == >) inc %bpointer
    elseif (%byte == <) dec %bpointer
    elseif (%byte == .) {
      echo @brainfuck -> $replace($chr(%mpvar),$chr(32), )
      bset &output $calc($bvar(&output,0) + 1) $iif(%mpvar,$v1,32)
    }
    elseif (%byte == ,) {
      if (!%input) {
        echo @brainfuck (Requesting more input) Type somthing to continue: 
        ;Saving status
        inc %spointer
        write -c datadump.txt
        write datadump.txt %spointer %bpointer
        write datadump.txt $bvar(&bfuck,1,$bvar(&bfuck,0))
        write datadump.txt $bvar(&mpointer,1,$bvar(&mpointer,0))
        write datadump.txt $bvar(&output,1,$bvar(&output,0))
        goto end
      }
      else {
        bset &mpointer %bpointer $iif($len(%input) != 1,$asc($right($left(%input,1),1)),0)
        if ($len(%input) != 1) var %input $right(%input,$calc($len(%input) - 1))
      }
    }
    elseif (%byte == $chr(91)) {
      if (%mpvar == 0) {
        inc %spointer 
        var %byte = $chr($bvar(&bfuck,%spointer))
        while (%kl > 0 || %byte != $chr(93)) {
          if (%byte == $chr(91)) inc %kl
          if (%byte == $chr(93)) dec %kl
          inc %spointer
          var %byte = $chr($bvar(&bfuck,%spointer))
        }
      }
    }
    elseif (%byte == $chr(93)) {
      if (%mpvar != 0) {
        dec %spointer
        var %byte = $chr($bvar(&bfuck,%spointer))
        while (%kl > 0 || %byte != $chr(91)) {
          if (%byte == $chr(91)) dec %kl
          if (%byte == $chr(93)) inc %kl
          dec %spointer
          var %byte = $chr($bvar(&bfuck,%spointer))
        }
        dec %spointer
      }
    }
    inc %spointer
  }
  haltbf
  echo @brainfuck Brainfuck output: $bvar(&output,1,$bvar(&output,0)).text
  :end
}