6p, I have not really formed an opinion on the subject yet.
On one level I would certainly love to see extra species, particularly small deer here in huntable numbers, as long as they do not cause damage to the exisiting wildlife.
Prof Dick mentioned above is an academic in QUB with an interest in alien species, he seems to think large number of Munties would do untold damage to the ecosystem. I am not entiely sure about that. Truth be told, large amounts of NI is an intensively grazed wildlife desert, and there is nothing there other than rye grass and thorn hedges.
Rural NI is also somewhat different to southern England, we do not have extensive broadleaf woodland, and only a relatively small precentage of farmland is arable. Most of NI is small fields bounded by thick hedges, average field size is well under 10 acres. I think Muntjac could live relatively well in this terrain. The down side is that rural NI is densely populated and there a lot of guys out lamping with 22 CF's. The Muntjac will be hard pressed to expand their numbers and range significantly if the lamp teams decide to hit them hard.