A 58-year-old grandmother has become the first person in Scotland to be convicted of illegal music downloading.

Anne Muir, a nurse from Ayr, was on Tuesday sentenced to three years' probation after she admitted downloading and sharing more than 30,000 music files – worth an estimated £54,000.

Muir was caught downloading the tracks after an investigation by two music trade bodies, British Phonographic Industry (BPI) and the International Federation for the Phonographic Industry (IFPI).

Strathclyde police searched Muir's Ayrshire home following a formal complaint from the two music bodies. Muir was arrested after the police found computer equipment that contained 7,493 digital music files and 24,243 karaoke files.

Muir pleaded guilty to breaching the Copyright, Designs and Patents Act 1998 in a hearing at Ayr Sherriff Court earlier this month.

However, lawyers acting for the grandmother of eight argued that she had not intended to download the music for financial gain, but to build up her self-esteem after suffering from depression. Muir will receive psychiatric counselling as part of her sentence.

According to research by Harris Interactive, about 1.2bn music tracks were illegally downloaded in the UK last year. The BPI estimates that those would have cost £984m if bought via retail sources, though it did not put a figure on how many sales were actually lost – that is, how many people who downloaded the tracks would have bought them instead.

The BPI and IFPI investigation into Muir began in 2008, when the music industry was pursuing an aggressive campaign to track down and punish alleged illegal downloaders.

The music trade bodies have since largely abandoned pursuing individual filesharers, focusing instead on lobbying governments on the issue.

A BPI spokesman said: "Today the court has recognised that illegal filesharing on a massive scale is a serious matter and has imposed a sentence aimed at preventing such behaviour in future. We would like to thank the Strathclyde police and the procurator fiscal service in Ayr for their diligent work on this investigation."

However, Loz Kaye, the leader of the UK Pirate Party, said he was "hugely dismayed" by Muir's "disproportionate sentence". He added: "The evidence should have been properly tested in court. It seems now there is a pattern of rights holders targeting vulnerable people to score quick wins for publicity."