Why So Many Sika Deer And So Few Red Deer?
I was asked this question recently, and the answer is interesting. Thanks to a park ranger for this answer.

Essentially it comes down habitat preferences of the two species. Reds are of the uplands (unless they can expand down into and exploit good lowland grazing, as they have in Killarney National Park), whereas Sika are woodland specialists.

The Sika have been able to take advantage of the increasing network of forestry plantations across the country, which they rest in during the day, and emerge from at night onto the often plentiful grazing which borders many plantations. So they’ve taken full advantage of the potential expansion of their range, provided by safe resting conditions and plentiful feeding for reproduction and growth. This has resulted in their population explosion.

The Reds on the other hand are generally more confined in range due to their upland status. The nutrition of the uplands is less than lowland pastures, so it tends to have a maximum carrying capacity too. However, in circumstances where they do have access to nearby quality pastures (say in Wicklow), their population can dramatically increase too as a result of the extra availability of food and nutrition. With the Sika doing the same thing in the same area, both populations near max capacity, pressure is on fronts, and bang… hybridisation can occur.

Although both species in Killarney National Park are/were facing close to similar population maximums, there’s no know hybridisation there and the culling in recent years has dropped the likelihood of it happening.

Both species are protected, meaning both require a licence to hunt. However Reds in Kerry only can only be hunted with a special Section 42 licence (in addition to a standard deer hunting licence). Reason being is that they are believed to be of native Irish stock, whereas (most) other red herds around the country were introduced at some point.