Northern Ireland Fire & Rescue Service (NIFRS) tackled three separate wildfires on Monday involving up to 200 firefighters.
The fires broke out at sites in Newry, Castlewellan and Armoy.
NIFRS area commander Mark Smyth said between 180 and 200 firefighters had been dealing with the incidents.
"They are reasonably significant in that all three of these incidents had a lot of resources at them today," he said.
By Monday night, all three fires - at Altarichard Road in Armoy, Flagstaff Road in Newry and Trassey Road near Castlewellan - were under control with a further assessment due on Tuesday morning.
Mr Smyth urged people not to "light any fires in the countryside".
"The operation is going as well as you can expect on a day like this with the amount of heat there is out there," he told BBC Radio Ulster's Evening Extra programme.
"The ground is very dry."
In a statement on Monday, the NIFRS said that since Friday morning it had "responded to 448 incidents across Northern Ireland, and that 171 of these have involved grass and wildfire".
"This has been an exceptionally busy spring period for fire crews who have worked hard in tough conditions to extinguish these wildfires," it said.
"Due to the recent hot weather, grass and gorse is extremely dry and NIFRS would appeal to members of the public not to light small fires or disposable barbecues as these can lead to significant wildfire incidents and damage to the countryside."