The answer is - it depends. In most years on established fescu lawns, there is little need for an irrigation system. The lawn may go dormant, but will usually recover well after a period of rain or cooler temperatures. Bluegrass lawns are not as tolerant of warm weather and tend to go dormant earlier in the season and may benefit more from an irrigation system, but I still don't consider it a necessity. If you are very particular and want a lawn that is at its best all summer long, a properly used irrigation system is needed. If you are looking to establish a new lawn, either from seed or sod, an irrigation system is a valuable asset in establishing your lawn.