A series which, by design, does not run regularly. It may be a series of annual made-for-TV movies, or a series composed of Mini Series airing every few years (But lacking the regularity of a Television Serial). Often happens when a Pilot Movie sparks enough interest to prompt a second movie, but not enough to prompt a regular series.
In the US, these are typically revivals of series which were originally regular in format. They are far more common in the UK. British Irregular Series are often aired on public television in the US as part of a Genre Anthology such as Mystery or Masterpiece Theatre.
- Columbo started life as a Made-for-TV Movie, then became a series which aired as every third episode of The NBC Mystery Movie. Later, it became a series of TV Movies which aired with decreasing frequency.
- The Rockford Files became a series of TV Film for a time after the regular series ended.
- Perry Mason did the same.
- As did The Incredible Hulk (1977).
- Inspector Morse aired as a number of Mini Series.
- Hetty Wainthropp Investigates did as well.
- And Second Sight
- And Adam Dalglish Mysteries
- And Prime Suspect
- Poirot in recent years.
- And in 2009, it temporarily switched to the "series of TV movies" format to allow the actors to follow up on other commitments that a full-scale TV schedule would conflict with.
- Rumpole of the Bailey.
- Rumiko Takahashi's Mermaid Saga, a manga variation of this, was periodically released in episodic stories over the course of ten years.
- Sharpe: Also ran like this, with a really noticeable time skip between the most recent installments.
- The British documentary series/social experiment Up takes this to extremes by airing a single instalment every seven years. It started by showing the participants at seven years old, and has gradually charted their lives in seven-year intervals ever since. The most recent one was 56 Up, which aired in 2012, meaning that in just over 50 years there have been only eight episodes in total.
- The German language crime television series Tatort. 1971 saw 11 episodes, 2008 saw 31, and not evenly distributed to boot. In the first twenty years, the length of episodes varied, too, up to two hours; more recently things settled on about one and a half hours.
- The Thick of It.
- Lupin III has made six regular anime series, but has run for over twenty years as an annual series of made for TV movies in Japan. Link to the list: Lupin III Yearly Specials
- Non-television example: When Tim Drake was introduced as the new Robin, he appeared in an Irregular Series before his regular series started.