How does -rt sound in S.Gaelic ?
Does it sound r-ch like "rich" without "i" ?
I think what you are hearing is a "sh" sound that appears in 'rt' and 'rd.' It is discussed in this pronunciation guide briefly: https://cuhwc.org.uk/page/unofficial-guide-pronouncing-gaelic
So the pronunciation becomes like "rsht." I think you will also like this page! It's a map showing that different areas of Scotland pronounce(d) words like "ceart" differently regarding the 'sh' sound.
The data in the second link was recorded around 70 years ago, often from informants who were themselves very old at the time, sometimes with limited proficiency (due to language shift being very advanced in some areas at the time) and often from dialects which are no longer extant. It was important research, but it's largely historical now and not really representative of the current (more homogenous) linguistic landscape.