I have dual citizenship with US/Ireland and will be travelling to the U.S. in February for which I have to renew the U.S. passport. If I renew the U.S. with my married name do I also need to renew my Irish one (even though there is still 5 years left), to reenter the UK.
It's not necessary for the UK. There are too many cultural variants of surname conventions and how they work for them to keep track of. A woman changing her surname to her husband's surname is one of many conventions.
It is necessary, however to prove that you are who you say are. People in your situation should travel with a copy of their marriage certificate and their prior passport(s). Those documents can resolve any challenges the UK Border Force might pose.
Finally, the Ireland Acts of 1948 and 1949 provide that an ROI citizen will be treated as a permanent resident of the UK so you would enter the UK under Paragraph 18 of the Immigration Rules with the single exception that the 2 year limit does not apply.