These seven islands all have permanent populations under 500 people with the exception of Fripp, which has about 900.
McMaster said he will decide Friday whether to ask people to leave the coast.
No other S.C. counties are under evacuation order.
According to the latest forecast, the SC coast is now less likely to take a direct hit from Hurricane Irma.
Top image: In 2003, high storm surge and wave runup during Hurricane Isabel caused widespread dune erosion in Nags Head, North Carolina.
McMaster delayed Friday asking residents and visitors to flee the storm until he received the 5 p.m. forecast from the National Weather Service.
McMaster said the state is prepared to open 200 shelters if needed.
"Sometimes there's a little bit of inconvenience to make sure you're being safe", he said earlier Friday.
Earlier Friday, McMaster rescinded a portion of an executive order issued Thursday calling on health care facilities and nursing homes to evacuate from coastal counties.
Traffic on South Carolina's highways is already heavier than normal with residents fleeing Irma's projected path.
Tropical storm-force winds also are possible Monday in some areas of the state, particularly south of Interstate 26 and in the western half of the Midlands. Evacuees headed to shelters should bring blankets, pillows and cots.
McMaster also said it's "likely" schools and government offices will be closed on Monday Sept. 11 and Tuesday Sept. 12 in some or all of the 46 counties. Several counties have announced closings Monday, including Charleston and Richland counties.
During a press conference on Thursday afternoon, he said though a mandatory evacuation has not been officially ordered yet, if people are able to, they can begin leaving coastal areas.
The county and its municipalities have taken action to prepare for Irma.
Irma is still projected to make landfall in Florida early Saturday as an "extremely risky major hurricane and will bring life-threatening wind impacts to much of the state regardless of the exact track of the center", the Natoinal Hurricane Center in Miami said.