From Roader's Digest: The SABRE Wiki
The origin of the term comes from the very first motorways which had a gravel soft shoulder. These were found to be inadequate when heavy goods vehicles stopped on them and were later strengthened, thus becoming hard shoulders. The name has stuck ever since, although other countries simply use the term shoulder or breakdown lane.
The standard width of a hard shoulder is 3.3 metres (approx 11 feet), but there can be numerous exceptions where conditions dictate. There should always be a solid white line demarcating the hard shoulder and this should be bounded by a line of red studs. Standard practice is to use a ribbed texture paint for safety purposes - this alerts drivers that they are straddling the marking and leaving the running lanes. These markings are sometimes known as a vibraline and were officially approved for use on motorways in 1987.
On earlier motorways, the hard shoulder was paved with a contrasting surface to the running lanes to provide a visual warning that it was not for driving on. This practice has largely ended, but can still be seen in Northern Ireland.
Recent developments in highway management have led to the hard shoulder being used as a temporary running lane on some stretches of motorway. This scenario is known as Active Traffic Management.
In the Republic of Ireland, a solid yellow line is used on motorways to mark the hard shoulder. They also may be used on all purpose roads including single carriageways, but the marking in this scenario is a broken yellow line. In Ireland it is customary on single carriageway roads with hard shoulders, for slower vehicles to pull onto the hard shoulder to allow faster vehicles to pass.