Out of universe, this seems like a possible writers' error, even though the lead writer on the film was Irish. Alternately, it may have been done to keep the film relatable for international audiences that are familiar with MI6 and not G-2.
That said, it would not be so implausible in the real world. MI6 is an exterior intelligence agency that primarily works to identify foreign threats. They have agents all over the world, including in Ireland. It seems plausible that they might attempt to get a high-value target from the Irish police. As the previous article indicates, the Gardaí almost certainly work with British intelligence:
“An Garda Síochána liaise closely with our security and law enforcement partners throughout Europe to share and assess any relevant intelligence and its potential impact on this jurisdiction,” the spokesperson said.
The Special Detective Unit (SDU) – an elite arm of the gardaí – has been liaising with its British counterparts and offering its intelligence capabilities to assist the spies with their work here.
In negotiations between allied countries over whose intelligence agencies get to interrogate and have custody of extremely valuable prisoners, I would expect the more powerful country to generally win out. For the same reason, it would not have been surprising if CIA agents from the United States had been given custody instead, as has happened in a few cases. These chains of custody can be quite complicated: for instance, someone
might be arrested by Thai authorities on the basis of British intelligence, handed over to an American intelligence agency, and later transferred to Libya for interrogation.
This is particularly the case if the more powerful intelligence agency has a decent excuse. Mulch is thought to have been working with Artemis Fowl to steal the following items:
Fowl is a known collector
and antiquities dealer,
but in dramatic new revelations,
he is also suspected
to have been behind
some of the biggest robberies
of the past 10 years,
including the disappearance
of the priceless Rosetta Stone
from the British Museum,
the irreplaceable 14th century
and the iconic Book of Kells
from Trinity College, Dublin.
Since the Rosetta Stone is rather well-known, MI6 has a good excuse to put pressure on the Gardaí or G-2 to turn Mulch over.