McGregor had to extremely dehydrate himself to step on the scale at 145. Look at the pictures below. The last two are Sodiq Yusuff and Alexander Volkanovski also weighing in at 145 lbs, but in California, which means we know that they rehydrated to 169.2 and 166.5 lbs the next day. McGregor is visibly more dehydrated, so he almost certainly cut more water weight than both Yusuff and Volkanovski. That means McGregor rehydrated to enter the cage around 170-172 lbs at featherweight.
At lightweight, McGregor's body fat percentage isn't as low, he enters the cage with around 4-5 more lbs of fat and around 1-3 more lbs of muscle compared to his featherweight days. That means he rehydrates to 175-180 lbs after weighing in at 155-156. He seems to keep his body fat percentage fairly low both in and out of camp, so his walk-around weight is likely 183-187 lbs.