There are 3 problems with your code:
(1) add the following lines before <script type="text/javascript">function initialize() {
<script type="text/javascript" src="http://maps.googleapis.com/maps/api/js?sensor=false">
</script>
This is in the post of your answer, but it is not in your file http://www.iol.ie/~murtaghd/map/map.htm.
(2) You need to call initalize()
(again: it is in your post, but not in the code):
<body onload="initialize();">
(3) Set the size of the canvas to get the map displayed (again: it is in your post, but not in the code). Afterwards you can change the size as you like.
<div id="map_canvas" style="width:500px; height:300px"></div>
You can see the site running on jsfiddle.
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