Only two units in the game feature this ability. Both are fairly common Normal Units, and both use a Ranged Boulder Attack. It is not possible to add this ability to any other unit in the game, by any method.
Accuracy over a distance is a problem that all physical projectiles inherently suffer from. Even today, with weapons capable of firing projectiles at mind-boggling speeds, aerodynamic stability and various other effects will eventually cause the projectile to veer from its initial course. The effective range of a weapon is determined by the most distant point where its projectiles commonly begin to display severe instability.
In Master of Magic, the Long Range ability represents a weapon with a decidedly-longer effective range than any common ranged weapon. Of course, given that magical ranged attacks suffer no penalties at any distance makes this somewhat less impressive, but Long Range weapons still hold a strong advantage over ranged attacks with regular projectiles.
As per the normal rules of combat, Ranged Missile Attacks and Ranged Boulder Attacks suffer a certain To Hit penalty based on the distance between the shooter and the target. Normally, the penalty is calculated as follows:
Penalty = -10% * (Distance / 3), rounded down
Therefore, if the enemy unit is 1-2 tiles away from the shooter, then there is no distance penalty for the shot. If the target is 3-5 tiles away from the shooter, the shooter gets -10%. If the target is 6-8 tiles away from the shooter, the shooter gets -20%, and so forth. The game manual states that the maximum penalty is -20%, but the Strategy Guide states that there is no limit. Currently there is no conclusive info on this issue.
With reduced To Hit, the Ranged Attacker will (on average) inflict less of its potential damage output on the target. Given that the target can normally also make Defense rolls to block additional damage, a Ranged Attack unit attacking at a great distance can end up causing no damage to its target, even with repeated attempts.
The Long Range ability's only function is to limit the above formula so that the resulting penalty is never greater than -10%. Therefore, the same -10% penalty is applied whether the target is 3 tiles away or 15 tiles away. A target at 1-2 tiles from the shooter still applies no penalty, as normal.
This allows Ranged Attacks to cause a little more damage even when used at a significant distance. Thus, such attacks are useful immediately at the start of a battle (when the two armies are still far from each other), where normal Missile Attacks don't fare so well.
Units with Default Long Range Edit
Two types of units possess the Long Range ability. Both are Normal Units:
The Catapult is available to almost all Races. The Warship, being much stronger, is available only to a few races. Note that the Warship's Long Range ability is one of the reasons why this unit is overpowered: it has virtually unlimited Ranged Attacks, and can move at a fairly fast rate, allowing the ship to maintain long distance from its target while pelting it with boulders.
Acquiring Long Range Edit
There is currently no known way to add Long Range to a unit that does not have it by default, whether by spells or through Magical Items.